Ging.He knew they could be wished back with the dragonballs dude. And by the way he protects earth all the time , and he led he's sons on that path too , that's a awesome father. What other father let's you have super powers bro?
^^This is the answer you all seem to have overlooked.Obviously Gohan. Goku doesn't spare the life of villains because he thinks all life is valuable, but because he'd prefer to fight them again even if their existence puts the populations of entire planets at risk. Gohan is more righteous than that and his anger is only directed towards those kinds of evil.