When you act on impulse you aren't thinking clearly. Your judgement is clouded by the emotion you are feeling in the moment. After you act on your impulse you may feel deep regret for something you may have said or done. You could hurt yourself and others. So you would want to undo your poor actions.
Utterson promises Dr. Jekyll that he will bear with hyde and get his rights for him and that Hyde gets justice. He also promises to help him when Dr. Jekyll is never there.
