It is certainly more useful than not killing him. By killing him, the total amount of hatred in the world decreases. Furthermore, all the money that would have been spent taking care of him in jail - feeding him, medical care, etc, will be spared. In prison he likely would have been involved in the Brotherhood. They have influence even outside of prison walls. By killing him he will not be able to further their cause from within prison.
There is <= 0 societal value to keeping him alive in prison. And there is >= 0 societal value in killing him. So kill him.