Similar argument can be used to justify the murder of many other people we would otherwise deem good. This is not the way. There’s a reason we no longer act like we did during the middle ages. It’s not for individual rogue actors to choose who gets to live and who doesn’t. That’s not a world anyone wants to live in.
Are you suggesting that that the nature of a murder victim shouldn’t be relevant in a murder trial? Killing a bad guy should be potentially completely mitigating. We have precedent for that. For example, Gary Plauché served no prison time for publicly murdering his son’s rapist.