Why not?
You know what? I was going to type a whole page's worth of an argument to show you exactly why you're not only incredibly wrong about the bolded portion, but also just plain ignorant, but then I realized that this is the internet, so it doesn't matter.
As for this right to life, I don't believe that it's a right, I see it more as a privilege, and one that should be revoked when you murder someone. In my mind, a right is something that cannot be revoked, no matter what.
As for the cruel and unusual punishment clause, I think that the death penalty would be a more effective means of punishment if it were cruel and unusual, as it would serve as a deterrent for the more rational criminals out there. The psychos like this kid, well, they won't be deterred by anything.