I do hope that that this Sand Man is Uncle Ben's real killer is just some ruse created by
Harry Obsorn trying to get at Peter Parker/Spider-Man
And Sandman was orginally not a killer. He was just a crook, and after getting his abilities, he just happened to hang around the wrong crowd (Sinister Six)
For a while after, he was apart of the Avengers being a good guy, but it just was not working out for him. One thing lead to another, and he was going back to his criminal ways.
Honestly, I am just hoping that they are leaving vemon up for a 4th film. Currently right now they got a LOT of story to cover in Spider-Man 3.
* Sand Man
* Harry Osborn/Green Goblin 2 (I hope they wrap that story arc)
* Gwen Stacy
who is going to die. I am not kiddding
* Spider-Man's Symbiot suit
Over all, they are putting a lot into the one film, have vemon in there for a main villain is just going to squeeze too much into the one film
BTW, that the 90's cartoon version of the vemon you have listed there, the orginal version of how vemon came around is here
http://www.spiderfan.org/characters/venom.html
This is where Spider-Man found the suit during the Secret Wars. And then ditchs the suit in a church. Only to have the suit find Eddie Brock.
The other more than likely, however less campy version of this is the Ulimate Marvel Unverse's Version of Vemon
http://en.wikipedia.org/wiki/Eddie_Brock#Ultimate_Venom
This is where both Eddie and Peter's perent where working on some cure for Cancer. However Peter accidently found out that the this black goop has other uses. When Peter found out that the suit was alive, he ditched it in a furnice.
Eddie was extremely angry at Peter for destorying thier parent's work. However Peter was unable to tell Eddie what really happened with out exposing himself as being Spider-Man.
And then this black groop found Eddie Brook, then along came Vemon after that.