'He was betrayed': Father of 5 executed by longtime friend with 9 mm Luger for allegedly having affair with his girlfriend after they took him in, cops say

Left inset: Mychael Thompson (KLAS/YouTube). Right inset: Antwon Watson (KSNV/YouTube). Background: The Las Vegas apartment complex where Mychael Thompson allegedly shot and killed his longtime pal and "roommate" Antwon Watson (KLAS/YouTube).

A Nevada man executed his longtime pal and "roommate" with a 9 mm Luger after accusing the victim — a father of five with another child on the way — of having an affair with his girlfriend, according to cops. The victim had asked to stay with the man while he was waiting to move into his own place, his family says.

"He went there to someone who he trusted, and he was betrayed," Faith Watson, daughter of victim Antwon Watson, told local NBC affiliate KSNV about her father being allowed to stay with his accused killer, Mychael Thompson.

"He knew my dad for a long time, and he shot him and left him dead in the parking lot," said another daughter, Hope Watson.

Thompson, 38, is accused of killing Antwon Watson on Feb. 12 in the parking lot of his Las Vegas apartment complex, located in the 4800 block of Spencer Street. A police press release says the two men "got into an argument that led to Thompson eventually shooting" Antwon Watson.

Authorities were called after Thompson's girlfriend, who had her three children inside the home, walked outside and found Antwon Watson lying near a dumpster, according to court documents obtained by local CBS affiliate KLAS. Thompson allegedly fled the scene before officers arrived but was tracked down through his vehicle and later captured.

Thompson's girlfriend told police he returned home around 7 a.m. after a night of drinking heavily and accused Antwon Watson of having an affair with her, according to the court documents.

"Lay off the alcohol," she recalled Watson telling Thompson, before the two of them allegedly went outside. The woman said she saw the men arguing chest-to-chest before they exited the residence.

"As [the girlfriend] walked out of the master bedroom, she heard one gunshot and could smell gun powder," the court documents obtained by KLAS say. "She observed Mychael in the bathroom and Antwon run toward the front door of the apartment. She went back to the bedroom and closed the door with the children inside. When she came out, both men were gone and she went out to find Watson laying on the ground and patrol cars arriving."

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Video footage obtained by police from a neighbor's security camera allegedly shows Thompson accusing his girlfriend and Antwon Watson of having an affair. The longtime friend moved in with them a month before the shooting, cops and family say.

"I hope he [will] be in jail for the rest of his life because … we're all in pain right now," another one of Antwon Watson's daughters, Destiny Watson, told KSNV. "We're all sad, the whole family."

After being taken into custody, Thompson allegedly changed his story several times and claimed at one point he wasn't at the apartment. Police said he wasn't "taking the interview and situation seriously" and refused to "give detectives his attention," according to court documents.

Thompson is facing a charge of open murder with a deadly weapon. He is being held without bail.