Prosecutors have a lot of leeway in terms of which crimes they want to prosecute and which sentences they want to pursue. A prosecutor, I'm pretty sure, doesn't have to prosecute someone who they actually think is innocent, and doesn't have to ask for a sentence that they don't think the person deserves (unless the sentence is statutorily mandated, which the death penalty never is)

Defense lawyers have to defend the people they're assigned to, but usually they actually value that service, even though they might not like the idea of people who have committed crimes getting out of punishments they might deserve.