That's construing it as murder though. It is moral for an individual to *kill*, for various reasons, and so it is moral for a group to kill for analogous reasons. A person may kill in defense of itself for example, and so a group may do the same. There's no logical inconsistency whatsoever.
A group of individuals sitting in a court room completely uneffected by the actions of the human they are sentencing to death are not under threat by that person. They are not protecting themselves. They are bureaucratically murdering another human that has not transgressed against them. It is moral for a human to kill another human if that human is protecting his/her life or someone elses' life against direct transgression. Likewise, it is moral for a group of people to kill in the same manner.
So a group putting individuals to death is just in some circumstances, just the way it's done in our society is unjust. I can agree with that.