Why be good?

[Iroscato]

Because the alternative would in all likelihood lead to the extinction of our species. From a coldly biological perspective that is the foundational reason. We know not what lies beyond the edge of death, and whether or not there is some supernatural being keeping tabs and taking notes on us all - no human being alive can know this either way for certain.
All we know for certain is that we're here, and our situation is what we make of it. We are stronger when we cooperate, we are stronger still when we unite, and we are at our strongest when we love. Genetic traits intermix with social constructs and form our civilisation as it is - though I'm of course talking about western civilisation, having been born and raised in it.
Many cultures across the world and throughout history have lived without the need to worship a god or gods, and many have been every bit as 'moral' as we are today. Others have worshipped gods and engaged in warfare, ritual sacrifice and slavery. It is a result of the multifaceted nature of humanity by which these different paths are taken.
To stress another point, and one that has always been at the heart of this question - are you saying you are only moral because you think you will face judgement for evil acts, and that's the only thing stopping you? Really ask yourself that, and be honest with yourself. You may find you answer your own question, for better or worse.