It's because he's a villain obviously. Plus he's also hell-bent on, in his case, "saving the world from the brink of ruin", by infecting the entire world using "Uroboros". (I recommend looking up Resident Evil 5 on the net, or playing the Resident Evil 5 game yourself (on either the Playstation 3 or X-Box 360 consoles) for more info, on why Albert Wesker's planning to spread Uroboros throughout the globe)
As for the "betraying everybody" thing, you can say that Wesker knows that setting himself up to be a "god", will require sacrifices.
I hope you find this answer useful.