Here's why:
* Forbear means to restrain oneself from doing or saying something, especially something that one has a strong desire to do or say. It implies patience and restraint.
* Punish means to inflict a penalty on someone for a crime or wrongdoing. It implies the opposite of patience and restraint, and involves taking action against someone.
Therefore, punish is a good antonym for forbear in the context of actions and reactions.