He claimed to be going to see his mother on Mothers' day (5 days ago in the UK). He may well have killed his own mother out of sheer self-righteousness and ego.
Do you have reason to believe that he didn't just mean "seeing his mother" literally?
I saw my mother on Mothers' Day. I stayed 200+ miles away from her while doing so. Facetime is a wonderful thing.
I'd be tempted to ascribe the self-righteousness moniker differently.