Evil: Monster or Mobster?

The concept of a scary villain is something I really take to heart. If a villian is either not believable, not bad enough, or just an idiot, it detracts from the hero. I recently saw the film "Batman Beyond: Return of the Joker", and it got me thinking. How do you make a villian scary? Where is the line between an antagonist and someone the audience truly hates, an abomination on a colossal scale? I'm going to use exanples of the Joker for this article. Take a look at the following clips (don't watch if you're squeamish):
The Dark Knight
http://www.youtube.com/watch?v=QehZjjwb7-I
Return of the Joker
http://www.youtube.com/watch?v=cKEfoC4tvJg
What is the difference between these two? The first clip portrays a man who is essentially a colorful bully. He kills the one mobster as a demonstration. For the most part, he is a complete sociopath, but he behaves in a manner like a bully. His whole plan is intimidation. Bascially, he is just like all the other mobsters in the room, except he takes more chances. I know that he at one point burns all of his money, but that is more because his goal is not to get the money. He wants to make Batman become a villain. His goal is to prove (as Batman himself points out) that everyone is bad. At first, this may seem really devious, but really it's the same maneuver as when a person caught for a speeding ticket claims that everyone was speeding (although that might be trivializing the crimes he commits a bit).
In the second, Mark Hamil's Joker is played quite differently. Instead of flat out intimidating the henchmen, he instead threatens one, lets him go, and then kills him when he relaxes. Not only that, unlike Heath Ledger's Joker, this one seems to actually enjoy it.
This is the kind of villain that disturbs me. He doesn't do bad things for power or to prove a point (as Ledger's Joker). Instead, we find a man who is a complete monster because he does these things for his own benefit (of course) but also plays it off as good fun. He's a lot more like Anton in No Country for Old Men in that he functions as a killer. He's not a bully, he's an absolute monster.
While the Dark Knight's Joker is evil, he doesn't scare me. If you watch him in the movie, he's an evil genius, but the way he manipulates people, it seems like he might easily be able to influence people. On the flipside, if you managed to figure out what he was planning, he tends to be easy to figure out (Once you find out he disguised his hostages as henchmen, his "hostages" are obviously the real henchmen).
The big difference in all of this is that if you are dealing with a real madman genius, he would likely go further than that, and in all probability do something unpredictable. That unpredictability is what can really be scary. It makes him all the more monstrous.
In short, a villian to me is scary when it isn't someone you can reason with. You can't predict it, and you can't stop it. All you can do is react to it. Essentially, this monster is a force of nature. There is no man behind his face, only the Joker.