For an atheist, moral absolutes are absurd (though still widely held). There are no more an absolute case for saying it's evil for a human child to play before a mouse before killing it, than it is for a kitten to do the same.

Rather, one could argue, morality is a set of emotions, norms, beliefs and rules we have gained through culture and evolution in response to certain behavior patterns. And these can be seen as having a game-theoretical foundation. The rules that assigns the label Evil to someone tends to select people that we (or our ingroup) are in an existential conflict with, at a level where it's either us or them.

For instance, we can have rules that define some patterns of behavior as Evil, and have different set of moral rules for treating people depending on whether they are classified as Evil or not.

Hmmm. Not necessarily. I mean if what you did does not change the outcome in any way, then is what you did even evil? If you have no marginal impact, can you ever do good or evil? I am not sure actually.

I realize that this could also apply to lower rank soldiers involved in some war crimes, etc. so I guess it is not that clear cut. But would you blame a random soldier of lowest rank for carrying out terrible orders and not risking himself(or his family)? Well I don't know. I guess I would not.

Of course, the stakes are rarely that high, but the harm done is also rarely that high also.