Malcolm Reynolds. In the past, he fought in the battle of Serenity Valley during the Unification War. He ended up leading about 3,000 men and most of them ended up dying. The battle was lost, which led to ... well, unification. He's got to have some guilt for that battle being lost, even if he can't really be blamed for that sort of thing.

Two members of his crew, Shepherd Book and Wash, also died... While Book wasn't actually part of his crew when it happened, Mal seemed pretty bothered by it. And he has to feel guilty about Wash, too, also because it's Zoe's husband.

He's not guilty, however, for all the thievery he does. XD;