Adding to my last comment: It seems to me that to write an anti-hero successfully one needs to be able to perceive different shades of morality. If you think it's perfection or nothing... then maybe you should be writing straight heroes vs villains.
I've always had a soft spot for the metaphor of colors, rather then just shades; the idea being, of course, that black and white viewpoints represent all-or-nothing good or evil, shades show that characters have some-good and some-bad, while colors are required to question the whole legitimacy of "good" and "bad" in the first place. Not everyone uses the same scale, and all; there are many dimensions by which to judge any action.
Indeed... but I would say that for a character to qualify as a real anti-hero, the author must present him or her as not measuring up in some way. I guess this does involve passing judgement.
Otherwise– in the worst case– the character can become a full-fledged Mary Sue, perhaps acting out the writer's fantasies of not being bound by laws/ethics/social conventions/whatever.