This could have been a good plot had the screenplay wanted to delve deeply into the terrain of conflicted morals; instead of taking it for granted that we accept that the events that happen are the result of a man following his only logical option. It's utterly manipulative and trite film-making, with everybody concerned thinking that they can just throw us some closeups of big doe eyes, and we'll suddenly forget that this is NOT action taken by those wanting to claim superior motivation.
If you're driven to pursue illegality, then chances are you're desperate, but do you have to be boneheaded about it, too? Why not try to get some of your colleagues onside in the scheme? In for a penny in for a pound, after all... That would of course eliminate your 'lone man in search of justice' angle, but at least it would make more sense, instead of feeling like a dupe, which unfortunately seems to be the practice of the whole tale.
"One Good Cop" is suffocatingly mawkish. Were it not for the violence and improperly used talent involved, it could easily pass for a dreary teledrama instead of a fully-fledged motion picture. It's trying to convince us that it has a singular approach; but the reality is that its type is nothing but bland and needlessly ubiquitous, which is a far more depressing snapshot of how life and the movie business works than any of the faux emotions tossed our way here.