Police need to be held personally accountable for what they do. "Good" police and supervisors that turn a blind eye or otherwise enable bad police to continue are not "good" even if they don't do bad things to people themselves. The idea of a few bad apples and good cops is nonsense. Why don't they uphold the law and arrest bad cops?
I would argue that the death penalty is necessary in this case because nothing will change until a very strong shocking and clear message is sent to police that the will pay personally for their misdeeds. Police impunity is the problem here. Not only does this impunity make individual justice unserved, but it erodes at the genuine authority and order in society. You protect a few criminal police and you deface all authority in the process. This policeman needs to be found guilty and put on death row, and the other three should get life. Supervisors should be charged with covering for them, prosecutors should be prosecuted for not prosecuting. It's the only way to save the system.