Compromised Coppers

Good cops quickly realise that tomorrow they could be in a life and death situation with a corrupt/evil cop behind them and if they have formed an opinion that the Good Cop is less of a threat to them if allowed to die or as the victim of a friendly fire incident, well...

So they join for all the right reasons but then find to do any good, indeed to just stay alive, they have to actively support the corrupt and evil. At least enough that they are reassured that their criminality is not threatened.