Another possible just-so story we can tell is that being (seen as) angry makes it safer to injure someone (e.g., "cold-blooded" murder or battery is seen as less acceptable than killing or battering someone "in the heat of passion"), so when we identify someone as incapable of retribution we're more inclined to make ourselves seem angry as well, the combination of which allows us to eliminate competitors while they're weak with relative impunity. (And, of course, the most reliable way to make ourselves seem angry is to feel angry.)

Is that actually the explanation for Raiden's reaction, though? Probably not; telling just-so stories isn't a terribly reliable process for arriving at true explanations.

Edit: Whoops... should have read drethelin's comment first. Retracting for redundancy.