Another way in which people with "conservative values" (i.e. "we hate porn") end up running apologetics for abusers (maybe not deliberately). Chances are he lives in a dysfunctional household (the parent certainly seems problematic enough). Also, even a framing of "he probably had a mental disability", and "the parent didn't really want to deal with that", would honestly be more likely than taking his arguments here at face value.
Also, slowly plotting and poisoning someone to death? I'm not really buying "heat of the moment".
Those are my thoughts though, if you have others, then I'd be interested to know.