I think the foil of the brothers' motivations can be simplified like this:
Ender is always doing the wrong thing for the right reasons:
Ender always wants to do no harm, but is often forced to harm/destroy by situations beyond his control. He does his best to do things in the most moral way, and for only the most moral purposes, but that's not always as possible as he would like.
Peter does the right thing for the wrong reasons:
Peter simply wants to do whatever is the easiest/most beneficial for himself, and is in essence sadistic. However, he becomes the world hegemon through peaceful means. To some, this might be misconstrued as a revision of character, but it is, in reality just the most convenient way for Peter to get the power he desires.