Dear Edis,
You've chosen a very controversial issue for the discussion. I believe we will read a lot of strong opinion statements here :)
From my point of view, it is absurd to punish a crime by committing the same act. I agree with you that serial killers are best punished when their freedom is taken away, but not for the same reasons as yours. Namely, if individuals are punished for taking a life, the state should abide by the same rule, because it is the individuals who run the country in whose name these acts are performed.
On the other hand, I can understand both economic and ethical reasons behind capital punishment.