The humans attacked mordor, and Sauron defended his home.
That's not quite how it went down. In the movies they even said prevented him from "covering the lands in a second darkness," which is basically what would have happened. He would have enslaved all the free peoples and ruled as an evil tyrant. I actually think he instigated that war, too, and the good guys were merely responding. In the books it goes more in depth, how Sauron was one of Morgoth's lieutenants and a vicious vampire thing.
That being said, I don't like the way Tolkien makes the bad guys all ugly and hideous. Makes them easy to laugh at when a flood invades their caves and tree people bash them together. In real life the villains and good guys aren't so black and white. Unless you're KKK.