Well since you started with why don't you define good? In the OP you say there is no reason to be good without believing in god - But before we get to that, we need to know what is considered "good", right? What do you consider good? Is it what says in the bible? What you feel god wants you to do? What the church priest/minister commands people to do? What your parents taught you?
Whoever fights monsters should see to it that in the process he does not become a monster. And if you gaze long enough into an abyss, the abyss will gaze back into you
