Shadow is one of the main villans of SA2, but is not inheratantly that way. He is Plagued by a past in which someone he loved died trying to save him. He feels he must get vengence on those that caused the death of that person he loved, and in his mind that is the entire population of earth. However, every so often he will remember the dying words of that person, and will, during those periods, actualy become more like a good guy then a bad one. He also belives himself to be the ultimate lifeform, as he has special abilities, one of wich involves taking over the power of the chaos emeralds to do a thing he calls "chaos control". He and Sonic are identicle in many ways, and Sonic ends up getting chased by the police a lot durring the game because they continue to mistake him for shadow.
Also, Sonic and Shadow are at odds most of the game because they both feel the other is an imposter of them.