Hitting your adult step-mom in self-defense when you were sixteen because she was coming at you with a knife is very different from an adult man grabbing a knife from his girlfriend and hacking her to death with it.
Also, at 16 you were a minor and your step-mother was an adult with authority over you in your home. This guy was the adult boyfriend who could have just walked out the door to his car and returned to his home.
No question he should not have been there. That was why I started with “from the moment she picked up the knife...”