Best Batman villain IMO. While I don't necessarily agree with mass omnicide, I can at least somewhat understand where's coming from with his nihilistic views on humanity and the multiverse (James Woods provides a great performance in the movie too). It helps that he's not a hypocrite and actually believes in what he preaches, unlike...say, Lots-o'-Huggin' Bear. He's also the perfect foil for Batman, having suffered similar tragedies but with different outcomes.
