In the movie Halloween (2007) why is the dude in the mask after his little sister?

i watched the trailer and i didnt quite get it.. can somebody explain why he went all psyco killer on everyone?In the movie Halloween (2007) why is the dude in the mask after his little sister?
He wasn't 'after' her. He didn't want to kill her. If he did, he totally could have and would have. He missed her and wanted to see her and be a family with her again I guess. But he went crazy because everyone was mean to him and stuff. Except the baby sister because well, she was a baby.In the movie Halloween (2007) why is the dude in the mask after his little sister?
I believe that Michael Myers if after is little sister because he was being tormented like crazy all of his life. He was finally fed up with it and just went Psycho. Basically, he is just a psychotic that wants to kill his sister.