Terribly sorry for your loss. It seems like your father was a very empathic soul. I am not going to try and say what his mindset was with any certainty when he made his decision, but, the fact that that was his nature perhaps had much to do with it. I can also relate to your grief on a certain level as my mom was probably the same. She always put others before her to the point of sacrificing everything she had including her health. She didn't die by her own hand, but, she did go slowly and painfully. Was horrible to watch and even though it was almost six years since she passed, it still affects me each and every day.
Anyways, I am rambling on. My apologies. If you need to talk, I will do my best to respond.
EDIT: I noticed your avatar descriptor is a Bill Hicks quote. Awesome. One of my personal idols.