My vote would go to Gaara. Naruto did have a rough past, seeing how mostly everyone in the village (xcept a few ppl such as hokage) hated him, but they hid their feelings and didn't hate him as much as to want to try to kill him. Sasuke did have a very sad past as well; Seeing his whole family/clan wiped out in front of his eyes in front of his brother, and couldn't do anything about it. But Gaara's past is too sad, I think I would go insane with hate if I was alone all the time, and everyone wanted me dead, and the only person that I rely'd and trusted in the entire world actually wanted me dead.