It's more moral than killing someone over a threat based on the pure conjecture that carrying out the threat will result in a punishment by a 3rd party.

Isn't the real threat the 3rd party that would be doing the jailing? Why is freely communicating what some people did a grounds for murder? He's not the one that is doing the locking people up -- it just so happens to be more convenient to murder him then to take on the justice system. Convenience does not make it the moral course of action.