I always thought that they tilted the camera angle when showing the villains because they were crooked - they were crooks.
That show was full of puns, and the crooked camera angle for the crooks was just another one.
-PJ
That's not necessarily off the mark, depending on the intent of the director, and the application of it to scenes only involving crooks within the story. A different take on the white hat/black hat motif.