“is there a issue of mesira when it comes to being a prosecuting attorney? “
No. Because we are in galut and can no longer enforce criminal penalties against other Jews within our own courts, we need to let the secular courts deal with the murderers, rapists, thieves, and swindlers in our midst. In some cases it is a matter of pikuach nefesh!
The only reservation that I’ve heard from any rabbi on this is that the death penalty standards in the US are so far below what is required by Chazal and Rambam for gentile courts that it is not permitted for a Jew to have anything to do with a death penalty case here in the US.