Are you saying you don't think he needed a stressor because he appears to be a sexual sadist? That's what I believe. He decided he needed "release" and went looking for a victim. Everything in this life could have been absolutely fine. He just wanted to have his world rocked. Unfortunately the way sexual sadists achieve that means a woman ( or a man or child) will be sexually molested in some way and may be killed.
No. I think BSL quite possibly has a mental illness as well as antisocial personality disorder. I don't know if he takes any medications but even if he doesn't I think he has worked at controlling impulses he may have. I think the stressors in his life made it difficult for him to cope and he could no longer control his need for gratification, much as an addict relapses or a dieter or alcoholic goes on a binge. I don't feel that it is a release but more of a need for a high - a feeling of pleasure- that he needed- a feeling of gratification, of power,of being in control because all around him everything else was out of control. (Dad being sick, GF family questioning RSO status).