That is one of the things I hated about the prequels. I’m all up for redemption and everything, but when you slaughter a school full of kids, I don’t think you get to come back from that.
In The Making of Return of the Jedi, the writers also had a problem with Vader becoming a Force Ghost at the end, but Lucas was adamant that even someone who committed atrocities could be redeemed. I very much agree with this.