Sandman's character is one of the few things that bothers me about Spiderman 3, actually. As far as I know, Sandman did not kill Uncle Ben. It happened more or less how we saw it in the original movie. The point was that Peter didn't stop the guy when he had the chance, and that led to Ben's death--not that it was really Peter's fault, but blaming himself for it is one of the driving things behind his transformation into a hero.

Who knows, maybe that's something they're throwing into the trailer just to be misleading, and later in the film they'll reveal that Sandman wasn't actually the killer. But in the comics, at least, he's more or less a morally complex character who is not such a bad guy.