The THREAT of punishment is what keeps people in line. Mikhail if guilty has already punished herself by leaving us. Another punishment would be half rations or execution, I dont see a need to punish a killer if that killer has no more reason to kill. Would you kill a jelous husband driven berzerk with rage due to rare circumstances? If no then you agree with me already, if yes then i'd like to know if you would kill all people with a potential to snap and go berzerk. I think it lies within all of us and a little civility goes a long way in preventing tantrum spirals. If you would kill every potential murderer then why not kill everyone else, what makes Mikhail so special, Killing once before does not in and of itself increase the chances of killing in the future, we are in no more danger than if we ignored the killing in the first place, she was a potential killer both before and after Norrecs death (as are we all). If you dont feel you can trust people around you what keeps you from killing those you dont trust. Maybe Mikhail had no trust for Norrec?, are you a potential murderer? Maybe we should all be kept in cages for each others safety? Am i making any sense?
I think we should determine who killed norrec, why , and determine if keeping them around is a risk we are willing to take. Exile is a solution already given to us by mikhail, but theres no need to loose another dwarf in her pursuit is what im saying, even if its only for a week or two.
As for future immigrants, we can THREATEN them with punishment all we like. Hell if it would please you i can engrave a rich history of punishment on our walls, lets say 15 came with us on our embark and 4 were killed for various offenses, maybe for not working hard enough eh? would motivating the immagrants like that please you?