What kind of deterrent could we implement that would stop this kind of thing from happening? The 'convict at all costs' mentality is so destructive to our justice system. And I really mean deterrent, not punishment. Yes, the bad actors need punishment, but I don't think the common response of 'make them serve the same time the victim did' is appropriate because that's just eye-for-and-eye thinking. That only serves to make the others stop doing their job at all, or get better at hiding their malfeasance.
When people call for an eye for an eye, it's evidence of frustration piled high. The frustration gets higher with every injustice.
The deterrent I'd suggest is, prosecute bad cops. What these cops did is illegal. If they're still alive, they should be prosecuted. And they did their crimes as employees of a public agency; that agency ought to be liable to be sued, and pay huge penalties. That would be a deterrent — if it happened regularly and loudly.
I agree prosecution is one step, but we've seen with death penalty crimes that even the ultimate punishment is not enough of a deterrent in a lot of cases. I don't like the solution being to sue the municipality because that only results in citizens paying the fine and/or losing services when funding decreases. I sometimes like the oft-floated idea of hitting the police pensions to pay the fines, but that's group punishment for individual transgressions. Having lived through that sort of thing in the military I can tell you it's not terribly motivating for the people who are actually doing their jobs properly. It typically just turns the rank-and-file against each other. Also, if we want good people to become and stay on as police officers then jeopardizing their livelihoods and retirements for things they had nothing directly to do with is not a great strategy to attract those people. I also don't agree with the argument that the 'good' officers that don't harshly deal with the bad officers are complicit and therefore also bad. We already have countless examples of good police officers being driven out due to raising ethical concerns or reporting issues. How do we protect them without ending up in the same situation we already have where it's almost impossible to fire ANY police officer?
Removing the scorecard mentality of district attorneys would be one huge step in the right direction but I don't understand the process well enough to recommend a concrete approach. Do detectives get measured the same way and that's why they do these things? Or do they just think they are infallible? I don't know. The complexity of solving these problems keeps me up at night.