In reality Sargeras is not really evil, he's fighting the shadow and trying to save the universe but in his mind, the end justifies the means. Azeroth itself is a world with a powerful world soul which is why Old Gods (void lords) are attempting to capture it and why titans are also fighting to have it not captured.
However, Sargeras found a simpler solution. Blow up Azeroth, problem solved. So right now we, the characters, are a weird combination that rose from all sources of the universe because all parts of the universe converged on this world which is pretty much the last stand on the universe. The main villains in the end will be the Void Lords and our heroes, characters or whatever you call them get more powerful by every day which is why Blizzard took the focus on our characters as time passes and in the end we'll be the one who decide the fate.