I agree with you if you mean that the individuals that first do the targeted group violence are the bad guys. But not if you generalize blame to the group.

Just about any large group of people, whether defined by by race, religion, nationality, or creed, will have bad individuals in it. And so the bad guys can almost always point to people from the other group that "started it".

When someone from the out-group commits an atrocity, it is just more proof of how bad that group is. When someone from the in-group commits an atrocity, it is an exception.

Groups can do bad things. Some groups exist specifically to do bad things.

Sure, and those groups almost always believe they are the good guys, fighting against some group that does and exists to do bad things.

The point is, if you are going to write of a group (presumably, the other political party) as bad beyond redemption, you better have a damn good reason to think that you are the historical exception.

Define "first", "violence" and for good measure "group". Do things ever reset, for instance? Are some wrongs too far in the past?

I mean the commonly understood contextual meanings of those words. Dictionaries are useful when you're unfamiliar with the words in context. I trust that you, being a native Anglophone, know what those words mean in the contexts they're used. However, if you're thinking of specific examples where you expect our understood meanings of those words to differ, I'll entertain those examples.

Also, violence isn't the only bad thing people do which justifies some sort of retribution.