No, you cannot compare emotional pain to physical pain. Where does that end?
Recently there was a thread on here where a man had killed someone who abused his daughter, and the overwhelming response was that there was no justification, regardless of what the other man had done. If someone abused or hurt or killed my child I think I would want to kill or seriously hurt them. While my own pain might justify the thought of doing so, it still doesn't justify the action.
Fact is that more people cheat on partners than don't, and yet we don't condone the idea of aggrieved husbands and wives going round assaulting the other men/women involved in the affairs.
I ask again, if the OW's husband had punched her instead of going round to tell the wife, causing her to have broken bones, would people be saying to her that it was no more than she deserved? That his pain was obviously so bad that he had no impulse control and couldn't help it?
If you found out that your child's TA had punched someone to the extent of broken bones would you really just shrug it off and say that she obviously couldn't help it and you were happy to be alone with your child? What happens when the kids wind her up to the extent she loses control?
Everyone has the right to be hurt over something which upsets them. But we are all responsible for our own actions. The reasons why the wife punched someone are irrelevant. All that is relevant is the fact that she did.