Possible homicide punishment?

My father was a victim of homicide in February. He was an alcoholic, and had Hep C which caused him to have a tumor on his liver. After drinking at a girlfriends house the girlfriends adult son punched him in the stomach, rupturing the tumor and causing my father to bleed to death internally. Here's the problem.. the woman and man say that my dad said to punch him in the stomach (I don't believe it) but my father did not like the man and had told us before that he did not like the way he verbally abused his mother. My father mentioned stopping it a couple of times. The cause of death is listed as homicide but the detective says that its just 2 peoples word against ours that it was harmless and an accident. (Of course they are the only witnesses because the 3rd person is DEAD.) My question is this " what could the possible outcome be?" The detective has given the case to the DA office and we are waiting to see if they can charge. I don't feel like this guy intentionally murdered my father but I really feel that it wasn't a playful pushing match. My dad was too "macho" to let somebody hit him in a weak spot .. The guy didn't even call 911 until after he came to my house and said my dad was unconscious. Can I sue? I don't want him walking away scot free.. He shortened the life span of my hero, my best friend, my daddy. He had a few years left and it was stolen away.