It is a subset of jury nullification. I am still stunned that anyone who read my post thought I was saying it was an actual legal term. I thought the post made it clear it was not an actual part of the Criminal Code. It's not a legally recognized affirmative defense.
In Texas, you typically see it when the defendant has a story that the jury will find sympathetic, but that fits no affirmative defense. The defendant has no legal leg to stand on. The lawyer does not come out and argue that the victim needed killing. It is more subtle than that. He will find SOME theory of defense, no matter how weak or invalid and technically use that. But he will present every bit of evidence he can to sway the jury and they may "accept" the actual legal defense theory on paper, but really vote not guilty because the defendant is sympathetic and they identify with his reasons.
Without going into too much detail, I once knew a man who became aware that a young teen girl he knew had been repeatedly raped by her stepfather. The legal system could have taken care of the SF and eventually, it did. But before he could even be arrested, this man beat the SF nearly to death. Knowing the defendant and his capacity to charm and that people just LIKED him, I predicted that he might escape a guilty verdict despite the fact that he made it clear he had committed the crime. I said that even if he was convicted, he'd get a slap on the wrist. And that is what happened. The jury was so outraged by what had happened to the teen girl that they convicted him of the least serious charge they could and he got a probated sentence. It wasn't "He Needed Killing" but it was close to it......"He Needed Beating to Within an Inch of His Life."
We often see it when something heinous has happened to a person and someone else commits a crime against the original perpetrator as an act of revenge/justice. What our law professors told us was that if we prosecuted criminal law, we WOULD see this defense and we had better be ready for it. If we practiced defense law, at some point we would get a client who had no other chance aside from this. It happens everywhere. We just have a term for it in Texas that is a little bit different.
It is not as if you see it often, but you see it enough that you recognize it when you see it.