A long-standing and intense family dispute over the custody of a disabled relative tragically culminated in a murder-suicide, claiming four lives, including that of a wealthy artist and her son.

Mackenzie Paul Williams, aged 45, was discovered deceased alongside his 80-year-old mother, Danielle Cuvillier, at her residence on Mercer Island, near Seattle, around 10:45 a.m. on Tuesday.

Prior to this, Williams had fatally shot his brother, Dominick “Nick” Williams, 34, and his wife, Harmony Danner, a 44-year-old physical therapist, at their home in Issaquah.

Following these tragic events, Williams traveled 14 miles to his mother’s house, where he ended her life using at least one of the 53 firearms in his possession before turning the gun on himself.

The precise reasons behind Williams’s actions remain unclear, although the family had been embroiled in a protracted dispute concerning the care of Nick for several years.

‘He was a loser, my brother in law who [killed] my sister,’ Danner’s brother Nathan wrote online.

‘He was in a custody dispute with his mother over his special needs brother… I was also told he was facing major heart surgery in a few weeks. He must have snapped.’

Nick suffered from Angelman Syndrome, which causes developmental disabilities, speech delays, and balance issues – and was completely reliant on his family for basic tasks.

Cuvillier was also ‘generously assisting both Mack, her oldest son, and his wife financially as they pursue their goals’, according to a friend’s court declaration.

Nick’s father, John Charles ‘Chas’ Williams, was his primary carer until his death on Christmas Day, 2018, which left Cuvillier to look after him alone.

The elderly mother did not take her husband’s death well, Williams claimed in a 101-page declaration to the King County Superior Court.

‘In the days prior to him dying she made several statements about how she couldn’t face going on and being the only one responsible for Nick’s care,’ he wrote.

‘She clearly stated several times that she intended to kill Nick and then herself after my father died.’

Half a dozen friends, along with his wife Danners whom he married in 2009 after dating since 1998, wrote supporting statements that now sound tragically misguided.

‘He is absolutely not a threat to Nick or anyone,’ Danners wrote.

Longtime family friend Morreen Hansen added: ‘Mack has never shown any signs of aggressive behavior. Mack is a diplomat, not a fighter.’

Williams claimed his mother had ‘serious anger issues and trouble relating to people normally’ his entire life and family friends cut them off because she was ‘too toxic and unbearable’.

Cuvillier allegedly slapped him across the face when he was eight or nine for ringing the doorbell too many times, but never hit him again because her husband said he would divorce her if she did.

Instead, he claimed she emotionally abused him and called him a ‘worthless sack of s**t’ when he was 14.

‘Danielle has been cruel and disrespectful, criticizing his wife, his health, and telling him he is going to die young’, and Williams would have cut her off if it wasn’t for Nick, his friend Chris Robbs claimed in a character reference.

By the time his father died, Williams and Cuvillier had worked out an informal arrangement where Nick would spend two days a week at his brother’s house.

But Williams claimed his mother’s behavior started ‘spiraling out of control’ in the past few years, with her ‘taking increasingly unpredictable actions that seem either out of character or very extreme’. 

‘My mother’s judgment and executive functioning seem compromised,’ he wrote – worrying she may have dementia.

He claimed she showed up to collect Nick clearly under the influence of alcohol numerous times, which Danner backed up in her statement.

‘You showed up to pick him up one night barely able to stand up,’ he wrote in an April 2024 email to his mother attached to his court declaration.

‘I had to force you to give me your keys and fold you into the back seat, then drove your car home with you sobbing uncontrollably the whole time which was very distressing for Nick.’

In another email, he admitted changing her contact info in his phone to ‘Queen Bavmorda’ – the villain from the movie Willow.

Williams claimed he also noticed a decline in his brother’s care when he picked him up for their twice-weekly visits.

Nick often complained of injuries or showed signs of self-harm, due to his condition, and increasingly asked to live with them full-time.

Part of Nick’s condition was a compulsion to pick at hangnails and rough spots on his skin, which if not monitored closely could become open wounds.

Multiple people who made declarations to the court used the example of a back injury from when Nick fell off a horse on holiday with his mother.

Williams claimed his mother didn’t get it properly examined until he pestered her for days – which revealed a closed compression fracture of L2 and L3 lumbar vertebrae. 

Williams also believed his mother was abusing Nick, after seeing her strike him twice while in their house during pickup.

‘She then completely lost her temper and planted her feet widely and began punching Nick in the arms, shoulders and back with a closed fist while yelling at him. He was cowering on the couch and trying to protect himself,’ he claimed of one incident.

He claimed Nick began to hit himself in the face when he thought he had been ‘bad’.

The feud came to a head on January 16 last year when Cuvillier was charged with fourth-degree assault over an altercation between her and Williams.

Cuvillier texted Williams at 11.03pm the night before to cancel their visit without explanation. 

She later claimed it was because of doctors’ visits, but Williams was convinced she had a more sinister motive.

‘I suspected that he had been injured and/or had engaged in another act of self-harm, and believed that the cancellation of his visit was an attempt by my mother to conceal this,’ he wrote.

Williams drove to his mother’s house first the in the morning and demanded to see his brother, but she refused.

Cuvillier alleged in her petition for a protective order that he barged into the house while on the phone with 911 and pushed her to the ground twice in front of Nick. 

Williams’ ‘finger was bitten by [Cuvillier] in self-defense due to [Williams] holding her down and constricting her airway’ and she suffered a black eye and bruising on her neck. 

Police arrived and arrested her after Williams told them she bit his finger. She spent a night in jail and Williams took Nick to his house.

‘[Williams] has a significant number of firearms at his house. His behavior has been unpredictable and aggressive lately. [Cuvillier] is concerned about [Williams’] access to these firearms given his recent behavior,’ Cuvillier claimed in her petition.

He surrendered 53 guns as part of the protective order – mostly pistols but also two semi-automatic assault rifles.

However, a March filing stated he was in noncompliance as there was one gun in his purchase history that was unaccounted for. 

Williams gave a different account of what happened at his mother’s house that morning, partially recorded on the 911 call.

‘Hang up the phone you f**king fatass,’ Cuvillier could be heard yelling.

The pair argued about Nick going with Williams or staying with his mother, until they suddenly came to blows.

‘She just physically assaulted me,’ Williams told the operator, which Cuvillier repeatedly denied. 

Williams soon after shouted, ‘Let go of Nick. Let go of Nick,’ and demanded his mother back up.

Williams claimed in his declaration that he pushed Cuvillier after she ‘balled up her fists’ and took a step towards him. 

He put the phone on the bed and Nick picked it up, so Cuvillier reacted by grabbing his arm and twisting it to try to get the phone. Williams pushed her away and she fell down. 

‘When she got up off the floor she started to attack me with full force, punching and clawing and slapping me all over my head and upper torso. I kept shouting for her to get back and shoving her away,’ he claimed.

Cuvillier tripped over and fell onto a couch, dragging him down with her on top of him. That’s when she bit his finger while yelling that she couldn’t breathe – though Williams claimed he wasn’t doing anything to restrict her airflow.

Both Cuvillier and Williams tried to present themselves in their legal filings as the best people to take care of Nick.

Glowing character references portrayed Cuvillier as a relentlessly dedicated mother who devoted her life to protecting and caring for Nick, resulting in development and socialization well beyond more Angelman patients.

Kate Maillot at Summit Community Center, where Nick attended regularly with his mother. Called her ‘the cornerstone of his success and well-being’. 

‘She has dedicated her life to making his life the best it can be,’ she wrote.

Cuvillier hired tutors and nannies from when Nick was a child to improve his speech. He can now speak in one to three word phrases, while most Angelman patients are nonverbal.

A neighbor also told the Daily Mail that Cuvillier and Nick were well known in the Mercer Island community and delighted everyone they encountered on their daily walks.

‘They were just such a source of kindness and happiness. Everyone enjoyed talking to them,’ they said. 

‘She was an amazing mom who did so much for her son.’ 

However, Williams claimed his brother repeatedly told him and Danner that he wanted to live with them, and his health improved in the two weeks he lived with them after Cuvillier’s arrest.

‘I would ask him if he wanted to go home and he would say ‘No, mommy bye bye’ and ask if he could stay with me,’ he wrote.

A video of Nick declaring he wanted to stay with his brother was included in Williams’ declaration.

Williams also included this text from his mother from October 2024: ‘Maybe he could live with you for a couple of weeks and you could see if you can make any progress with the problem. 

‘I told him that he was making it impossible for me to live with him. He said he wants to live with you.’

Williams claimed his mother filed the protective order ‘so that I cannot be a witness to her abuse and mistreatment of him’.

‘I believe that this court has been cynically and intentionally used as a weapon by the petitioner to take away not just my rights, but my brother’s fundamental rights to self-determination and dignity,’ he wrote.

Nick was in his brother’s care until January 30 when the protective order sent him back to his mother.

Williams and Cuvillier spent the whole of last year in litigation before finally hammering out a binding agreement that would let Nick visit his brother twice a week.

The protective order was dropped on December 10 and Williams’ gun collection returned to him.

Just three weeks later, everyone involved in the tragic saga was dead.