There is a difference between reacting to a situation vs creating a new situation.
Very few people would argue against having to use violence to stop someone else from using it, in the moment where other options don’t present themselves. However a murdered container in prison is no longer a threat. The state has the luxury of just keeping them there until time and nature does her thing.
Basically the rules for a crisis are not the rules for a non-crisis. Additionally, if it is required to use violence to stop violence at least the hope is something bad won’t happen. Not the case for someone in jail. The bad thing already happened.
More broadly Ukraine has the right to defend herself. She does not have the right to burn down parts of Russia 40 years from now when the war is long over.