Ambition can be good or bad
1. Not all characters are corrupted by their ambition, as Macbeth and Lady Macbeth are. Ambition can be a positive thing if it's motivated by a desire to help others rather than yourself.
2. Malcolm and Macduff are ambitious for their country. They want to take Macbeth's power away from him, not for their own selfish desires, but for the good of Scotland.
3. Banquo is ambitious for his sons - he hopes that "there come truth" from the Witches' prophecy - but he doesn't act on the predictions in the violent way that Macbeth does.