Personally I only see motive as important when the actions a person has are good, Thanos thought he was saving the world but it just made him seem stupid that he didn't think 'hey with infinite power I could just double the resources in the universe and get the same result'. Motive is how you make the Protagonists rival bad, by making them only do good because it gives them money or what not. But and here is the important part that is my opinion on how I like things to say that it is impossible for a villain to have an interesting motive and be turned to evil because of it would be presumptuous, which is what annoys me about people saying villains need to think they are the hero, that's what they like but not the only way to make it work.
I found him to be a bratty teenager with no emotional control that had no right to be a master of the darkside, and wasn't threatening from the instant he took his mask off, but that's me. On terms of the concept of someone modeling themselves after a former dark lord, I think that the whole: Morgoth - Sauron - Saroman dynamic might be more to your liking.