@MyPaw I believe he did need to exact vengeance for two reasons:
1. He was risen by his grudge against those traitors. Failing to exact vengeance would be going against his very nature and would risk breaking the magic that makes him exist.
2. He still would need to destroy the kingdom because he needs to rise as a threat and the Valley of the Dead is the absolute center of his operations. He doesn't have any leeway on choosing where to settle so taking over the capital and the nation to form the danger counterweight he was talking about was imperative to his whole plan.