In all fairness, Venom isn't that much of a two-dimensional character. In fact, I'd venture the opinion that he has a very complex identity. There are a lot of angles you could use when approaching him in a film. The issue of suicide, the duel personality and relationship between he and the symbiote, the consequent struggle between justice and revenge that occurs in him. The idea that Venom is the anti-Spider Man, a lethal protector who straddles the line of good and evil would have made for a very compelling storyline.
Spoiler:
Except Marvel already have a similar character: The Punisher, and he BEGAN as a Spider-Man villain. Venom is much better off as Pete's psychotic stalker.