What made this movie a let down for the majority of the fans?

[WorthyStevens]

All quite true. Also, if Sandman accidentally killed Uncle Ben, and was remorseful, and more than likely realized Spidey was after him for that reason, why did he even team up with Venom to begin with?

He didn't have any clue that Uncle Ben was Spidey's uncle at that point...

If you were him and needed the money, why bother fighting Spidey (especially if Venom already was planning to) and just rob some more banks?

Perhaps because Spidey just tried to kill him. And considering that Spidey will probably keep on trying to do so, I think it's safe to say Sandman needed him out of the way to help out his sick daughter.