I dare say he might be jealous and covetous / resentful.
But no excuse for murder. As she showed him around the spacious, bright and airy dwelling with white, terracotta cornice ceilings, fully-fitted kitchen and ample storage areas whilst positioned conveniently for local shops and cafes he couldn't help but contrast his plight from that of his sister's, toiling away on farmland out in the Styx in a tiny cottage he himself didn't own. He peered into Nicholas and Daniel's bedroom, offspring that he himself would have welcomed, now only coveting his sister's and observing his parents' reaction to their grandchildren and comparing that to his own loveless upbringing and how they had driven him away from Suzette and his own ersatz family.