Avi K: A Ben Noach could get death penalty throgh a ruling of one judge. Look upin Gemara and Rambam.
But anyway the question here was if al pi torah it is allowed to kill a goy, so my answer was that a goy killing a goy is from the Seven mitzvos Bnei Noach, and that there is death penalty for it. I was not talking about giving actually the death penalty in our time.