The people who first do the targeted group violence are the bad guys.
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.
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.
Also, violence isn't the only bad thing people do which justifies some sort of retribution.