Is it normal for my father to feel really guilty over his service during war?

My father is a veteran of the Gulf War. He was in the USAF as a navigator. Even though he has no proof, he believes he's killed people in Kuwait/ Iraq. He feels such guilt that every Memorial Day, he thinks about the mothers, wives and children left behind (on the other side) and it sours him. I know most veterans pull a trigger or throw a grenade might feel like this, but I don't know if my dad should hurt this much. Is there any place to help him or does anyone have advice to deal with this? Thank you.