NASHUA, N.H. (AP) – A man accused of killing his girlfriend’s mother told police he knew that doing so was crazy and he didn’t entirely understand it himself.

William Sullivan 20, of Willimantic, Conn., is charged with first-degree murder in the beating and stabbing death of Jeanne Dominico two years ago.

During his trial on Friday, Detective Sgt. Denis Linehan testified about Sullivan’s interviews with police. Sullivan said during one point during the struggle in Dominico’s home, he thought that if he just stopped and walked away, perhaps she might just forget the whole thing.

Sullivan also spoke of reaching a “point of no return” while arguing with Dominico, and remarked, “Once things escalate, I think too fast … that’s what the medication is for.”

According to the interview transcript, “It happened in like a split second. … I just lost it, I don’t even think I was thinking,” defense attorney Paul Garrity read. “I’d have said I wouldn’t have done it.”

Sullivan’s lawyers are arguing Sullivan was insane and incapable of forming a plan to attack Dominico due to his mental illness.

Sullivan’s girlfriend, Nicole Kasinskas, 18, pleaded guilty to a reduced, second-degree murder charge. She may serve as little as 35 years in exchange for testifying against Sullivan. Prosecutors say the two plotted to kill her mother because they wanted to live together and she opposed it.

Confronted with information Kasinskas had given to police, Sullivan told detectives about earlier attempts to sedate Dominico by putting allergy and cold medicine in her coffee. He also talked about setting fire to her house in hopes that Kasinskas would move closer to Connecticut, where he lived. Sullivan told police he ultimately hatched “the perfect plan” to kill Dominico and make it look like a robbery.

The trial isn’t expected to be finished until some time next month.