Even if his parents had a bad relationship with him they didn't intend to make him a murderer.
tit parents exists everywhere and most people don't turn out to kill someone

I also think there's the consideration that even if he did have a poor relationship with his parents, that's still a huge shock to them. Just like it's a huge shock to his MIL and other extended family probably. I doubt any of the family is sitting there going 'oh yeah, he grew up to just kill someone!', if anything they're probably noticing small things that now look very very different in a new context