I like the character of sandman, and the effects in the trailer regarding him certainly looked good- but I agree that doing a 180 and making him the killer is just plain ridiculous. Obviously the goal of this movie is to make spiderman as big a dick as possible, instead of being the goody goody he was before, to give the suits effects on him a foundation. I mean, come on, spiderman would have chased down the sandman anyway, this quest for vengence is completely unnecissary.
I suppose another reason they could have made him the killer was to give themselves a plot. Cause quite frankly, it seems like the whole story of this movie could be fit into one of the 30min (well, 20, if you count commercials) episodes from the cartoon. That kinda scares me, cause that means there will be a shitload of "character development" (aka, spideys quest for vengence, parker's friends and family's lecturing on morals and responsibilities, and emo-ness (most likely from Harry, followed by parker near the end of the movie)
Despite those gripes, I still think overall it will be a solid movie, and I am certainly looking forward to it. Hopefully #4 will be based on something more substantial.