No, this is wrong. And yes, he is as horrible as can be.

It is not ours to play God. Vengeance is not ours, it is God's. We get dragged down into his evil doing this.

You might argue the need for deterrence, but even that doesn't justify taking a life.

I have to admit that it is incredibly tempting to go along with this given the cowardly way in which he took down his military brothers who accepted him in, and that horrible damage that he inflicted upon the families involved.
But it is still wrong, and evil things are usually tempting.