What is scarier, a psychopathic killer or a shark?

Recently I was reading an article about the new Batman movie, The Dark Knight, when director Christopher Nolan said something that really got me thinking. He said that in the new film there will be no explanation of why the Joker is a psychopath, because things are scarier when they are unjustified.

He also used a great example to back this claim up. The shark in Jaws. As an audience we all felt the uneasy feeling when the music kicks in and that infamous fin appeared near a beach, but apart from using nature as a reason, we cannot justify why this is. We don't know whether the shark was abandoned at birth, treated badly by its parents, not given something it asked for or any other psychological reason. The shark just appears, causes mayhem, kills an innocent swimmer/ surfer and then dissappears.

Linking like for like, when Tim Burton made his Batman movie, certain elements of the Joker's back story were adjusted to give him an emotional link to Bruce Wayne/Batman. He made Jack Napier (the Joker) the killer of the Wayne family. Even though we know some of this background, Burton never explained why the Joker is the way he is.
I read a wall post online the other day that stated that Jack Nicholson's Joker acts in this way because of the accident that bleached his skin, an accident which he blames Batman for. Technically this is true, but Tim Burton also shows Napier to be a murderous madman before the accident, so again, we are left without an explanation for this behaviour.
This could be intentional, using the same principles of Nolan in that things are scarier when we don't understand what created them.

I'm not sure where to stand in this arguement. I can see the point of view that the unexplained is scary, which is why supernatural horrors and thrillers are good at dishing up that sickening nervous feeling. But I also fear the twisted logic that could turn an everyday human into someone capable of an atrocity. At the same time, does this logic qualify as an unknown?

But there is a last minute piece of evidence for the arguement that the unknown is scarier! Cloverfield!
Cloverfield is a brilliant concept and the idea of the handycam style footage does wonders for cranking up the sense of fear and panic. This film drew me in and I really began to feel that I was part of the survival mission that the characters were undertaking. But, this changed the minute the monster is seen. From that point on I felt deflated and I no longer felt the same heightened sense of fear. If the movie had run its course and never shown the offending creature, I cannot help feeling that the audiences would have been left thinking "what have we been running from and what kind of creature could cause this kind of destruction", rather than "wow, another CGI monster".

I think the arguements on the debate could run for a long time. But all I know is that in a society where movies are becoming more and more predictable, a bit of the unknown would not go a miss. If there is fear due to lack of understanding, its hard to know what to expect and predict what is coming.