It's not the prosecutor's job to decide if the law is just or if the punishment is commensurate. The prosecutor's job is to prosecute. Unfortunately, a primary tool in that job seems to be plea-bargaining. I hope that some change in the way plea-bargaining is used comes from this tragedy, but it's disingenuous to get upset when that reform fails to arise from the very people who rely on abusing that tool to do their job and who in principle have no particular say in how the law is written or interpreted.