I think Voldy killed his father for more human reasons for forgeting about him and his mother, for leaving her to died and it is not like he could have supported them even if he had no contact with them. He was rich and a snob who had he been a wizard would have been in Slytherin. Voldy is more like his father then he would want to claim.
perhaps he killed his family so to help his goal of becoming immortal but i tihnk he killed them for just not careing about him or his mother. A normal human being do not just marry someone and leave them in a world(Muggle world) which they have very little understanding! i am guessing Mrs Riddle was a pureblood witch how turn her back on her family for the love of a muggle who betrayed her and left her and her son to died!
Sorry if am ranting but I hate people who betray others they and there lack of feeling hurt so any people!