In-game morality (just like real world morality) is a generalization of good/bad decisions that are usually passed on second-hand. In the real world, it is considered morally right to help another person if it doesn't put yourself out too much. If it cost you a lot, then there is no overall social gain, just a transfer, and thus it's no longer reguarded as automatically the 'right' thing to do. Or again, a starving man can steal a loaf of bread from the grocery store and face very little penalty. But a girl who shoplifts for no other reason than she doesn't wanna pay for it is considered deviant, and in need of correction.
Such rules of behavior can exist in a video game, and we can rightly call them morals. For example, killing anything that moves is a moral: If you don't, then you will die; moral of the story? Kill anything that moves.
One difference that I see between IRL morals and In-Game morals is that real-world morals often have to do with behaviors that effect other people, while most games these days are solo affairs which offer little conception of survival of the group. But there are exceptions. I've seen people in League of Legends tank a tower hit to help a squishy ally get past it.
But that's not a behavior I've seen in Diablo 3 or Borderlands, even though they claim to be team oriented.