Being a good cop just isn't nice in the moralistic sense. It also allows you to progress through the entire story. If you are a really awful cop, then when you reach the first 'branching' you can only choose the bad cop pathway. Only good cops can progress forward to see what happens in the story. The same happens at the second branching, only medium-good cops can keep going ahead. So if you're a great cop, you can see both the bad AND the good endings. But if you're a bad cop, all you will ever see is that side branch into 'badness'.

True Crime - Streets of LA Walkthrough


True Crime - Streets of LA Review