Chandler, Ariz. ? An 18-year-old high school student caught with his 48-year-old math teacher in her bedroom was stabbed to death by her boyfriend, who was himself a former student of hers, police said Tuesday.

Chandler police said 20-year-old Sixto Balbuena told them he never meant to kill Samuel Valdivia. He allegedly told police “the blade went in like going into butter” and that he just wanted to show Valdivia how much he hurt him by sleeping with Tamara Hofmann.

Balbuena, a Navy sailor on leave from California, was arrested on a charge of second-degree murder after police found him covered in blood and he told them about the killing. He remained jailed in lieu of $100,000 bond on Tuesday and did not yet have a lawyer.

Balbuena found his girlfriend naked and Valdivia in his boxer shorts in the woman’s bedroom around 2:40 a.m. Friday, according to police reports and court documents.

Balbuena told police that Valdivia apologized to him before Balbuena began kicking, punching and throwing things at him, according to a police statement.

Police said Balbuena told them he “wanted to teach the victim a lesson,” and stabbed him in the lower side with a kitchen knife, according to the court document. Valdivia later died at a hospital.

He also threw Hofmann to the floor jumped on top of her and demanded to know how long she had been cheating on him, the statement said.

Police said Hofmann taught Valdivia math at El Dorado High School in Chandler and was also Balbuena’s teacher when he attended Marcos de Niza High School in Tempe.

Police spokesman Sgt. Joe Favazzo said Hofmann is being investigated for potential misconduct relative to her involvement with Valdivia.