If this person was freed from prison and then the relatives of the two dead people killed him in retaliation, how much prison time do you think it would be just for them to receive? Deterring this kind of informal retaliatory murder is one of the jobs of harsh sentences in the formal criminal justice system.
The relatives of the dead people could probably have the convicted murderers dealt with in prison if they cared enough about it. They did sue the murderers (and likely won by default) for $10m for each of the two murdered men. As the convicts have life sentences and no income they will probably never even earn back their legal fees.
I think it is harder than you think it is for people who aren't already connected to the criminal underworld to order a hit on someone in prison. But I think the desire for revenge is muted if a person is being legally punished for their offense by being locked in prison at all, as opposed to being free in public.
