Someone who would suspend habeas corpus isn't devoted to democracy. Maybe the idea of democracy but not the rule of law on which it depends.
And as for a dictatorship being justified in his mind, which dictator hasn't been? As a wise man once said, man isn't a rational animal, he's a rationalising one. Our capacity for self-justification is limitless.
So given the right circumstances, Lincoln could start acting like a dictator?