How would you identify Slade?
Personally I think of him as more of an anti-villain as though his morality more immediately identifies with "bad guys" or villains, he doesn't act out in the name of evil but rather following 'his word is his bond' principle. In this sense Slade acts according to a code; a standard he sets to regulate himself. It's not a completely immoral code as his goal isn't the destruction of worlds or people outright, but it's certainly fairly murky. Still, he will team up with protagonists to take down common enemies. True villains would never rationalize doing such things unless it involved foiling the heroes' plan. Anti-villains and anti-heroes, however, don't mind doing mutually beneficial things if its more logical for them. Anyway, that's my feeling on it.