Originally Posted by Wuloric
Morg's "evil" is pretty small-scale, simply wanting to best Kayle in the League and go home to be praised as the superior sister.
I say Malzahar, for mostly the reasons people have already stated: Even if he is not the true leader of the Void, we don't have the true one, and he is still the general overseeing it until his masters appear. Cho and Nocturne may as well be his monster pets he let loose in the meantime.
Swain and LeBlanc are iconic evil, in that they are enemies of Ionia and Demacia, but they too would fight against the Void. Hell, with champions like Riven around who realize there is such a thing as "too much evil", despite being loyal to Noxus, would even team up with the "good" nations to fend off the rest.
Shadow Isles champs (Morde, Hecarim, and I guess Karthus?) seem even more neutral in the fact that they enjoy power over their opponents rather than power in general; no opponents, no power.
Poor Renekton, getting his ass kicked by his bro both in-game and lore-wise