Just cause he gets beaten doesn't mean he's done for. He could get knocked out by Harry doing some crazy kamikazi crap to kill him. It shows Venom sprawled on the ground, cuts to a touching goodbye between Harry and Peter (enter: Cause you're my best friend, Pete.) Then it shows where Venom was laying again and he's gone.
You know? One of those cinematic cliches.
Ok. So i think the movie villians are gonna happen like this:
Spider-man 3: In the beginning John Jameson brings the symbiote back from space, crashing in NY city. Spidey saves him, symbiote gets on him. He wears the suit for most of the film.
During the movie it shows the back story of Brocks hatred growing for Parker. Spiderman fights Sandman as the main villian in the first 2 acts. Harry Goblin fights Spidey in the final showdown. At the end of the film Spidey gets rid of the symbiote in the church and a black shadow comes over Eddie, who was about to comit suicide.
This would be an awesome cliffhanger setting up for 4 with Venom as the villian.