It's one thing to let go someone who tried to rob the bird but failed, compared to letting go someone who succeeded at robbing it but still came back to cause more trouble, even threatening to kill people. Frieren is such a good person she only kills demons/monsters, but that group would deserve to have someone die. They should have just made themselves scarce after managing to snatch the bird. That one scumbag even repeatedly talks about killing the kids. Of all the mages so far, Richter is the most derserving of death.