just because something has the potential to be something else doesnt mean it should have the full rights of what it has the potential to be.
revenge serves no purpose, if you look at anyone who makes policies dealing with the death penalty or anyone who is to be taken seriously about defending it, they will reject that revenge has any part in it.
revenge is a somewhat immature-minded concept, george orwell put it good
Revenge Is Sour
I doubt this is a common thing, there are rare occourances that happen in this world, but this isn't common therefore not a good example
its not about granting rights to serial killers but its about that we dont have the right to take their life away
but regardless, this is a completley different subject, don't try to mix the two