I WOULD LIKE TO SOLVE THE PUZZLE.
The idea of "genocide" and "mass murder" or "mass casualties" is not one and the same. By definition, "genocide is the intentional destruction, in whole or in part, a national, ethnical, racial, or religious group". Thus, killing off a set of people must be done with the motivation/intent that can be linked to one of those types/subjects.
So, if I kill 100 people, it's just mass murder. But if I kill them because of their religion and say I want to kill every person that goes to that church,
then it's genocide. There are plenty of examples of IRL genocides on Google that I am going to assume you are either aware of or have the capacity to go look on your own time of your own volition.
EXCUSE 1:
In writing, the need for genocide is indeed generally validated by one of three ways, two of which you've described.
- There's a severe and real risk to my loved ones.
- There's a severe and real risk to myself.
- I need to do this to achieve a goal.
Most authors will utilize one of the first two in order to commit their genocides. As you mentioned in your opening post, it's an easy sort of "gimme". The problem, as you also said, is that most of these individuals are so strong that the threat isn't a
real threat, as they could honestly squash said enemy at any time. But, it's a
perceived threat. Perception is reality and individuals are known to act on these perceptions without much thought. Why? Because we're lazy and it's easy.
The issue becomes then the "why". Why bother? If the MC is so strong, why do anything at all? Because generally in these instances, the MC has developed their power through a number of trials and understands the idea that they must act first rather than react. By being proactive in their attack of the enemy, they can safely protect their loved ones without getting them involved and without the headache of having to save/help them later on if they wait.
Authors usually fail to describe things this way or fail to verbalize it because either they themselves don't realize it, or they do it purposefully to keep the Reader attached to the MC. Maybe the MC doesn't make these thoughts out loud or the world is developed in such a way that it's an inevitable occurrence. Either way, they either make the move and do the killing or they don't and someone they care for gets hurt.
Threatening people is one way to alleviate having to fight them but in the case of most bad guys, intimidation only goes so far. If the group is developed enough to give the MC, who's OP, enough pause to warrant attention, then intimidation isn't going to help. The villain will always try to find the MC's Achilles heel and then proceed to exploit it. And why wouldn't they? They're the bad guy for a reason.
The third option is more rare as it's something that by this point is likely either justifiable for the MC to become stronger/continue to develop, or it's a turning point in the story. Genocide isn't a "good" thing. Unless it's of a truly vile threat that everyone in the story's universe agrees can be killed without issue, then genocide of any kind of enemy will bring about a negative connotation. Thus, this option is usually only for the MC's who are fighting hundreds of monsters/demons or are leveling up and need a shit ton of EXP. It's usually not negatively motivated but needs to happen for progress. Hence why either it's not seen often or we actually don't notice it.
What is it, the manga where the chick killed a million slimes to level up or whatever? That was genocide of slimes, but no one batted an eye. Why? Because they're slimes and it was in a game world.
But, as I mentioned, there's also the potential for this to be a turning point. Maybe the MC has already shown signs of being an anti-hero, and now genocide is on the table to achieve some sort of end. If he follows through, he's gonna have to carry that weight. If not, maybe a portion of the people will die anyways or whatever event is about to happen may or may not still happen. Instances like this where the MC is cornered, they make a decision. Some will slap the button and kill people. Others won't and will have to pay the price of that choice, whatever it may be.
EXCUSE 2:
Morality is always a factor in genocide and any negative action we perceive to the negative. We always have to consider the "why", the intent, the motivation, the background of the MC, the circumstances, and the outcome that may or may not happen. Naturally, non-human MCs can more easily justify mass murder because they don't care about humans, regardless of whether they are an ex-human or not. Their emotions and motivations will always be different.
Failing to accept that fact pushes your own burdens on the MC that they don't have to follow. It's like adding a rule at the bottom of a test but saying it's an opinion based rule that won't affect the score- no one cares but you and no one taking the test has to or will care about it. Humans who become non-humans no longer have to follow the human rules. Why? Because they aren't human anymore.
EXCUSE 3:
Light-hearted genocide is indeed less of a genocide problem and more of a scale issue. This one is less trying to downplay the genocide bit and more failing to understand the scale of what the character did or what actually happened. Of course, it's still pretty shit on the author to not pay attention to these details and makes the characters look silly but that's the honest answer. The genocide isn't the problem, though.
IN CONCLUSION: Genocide is a skill issue authors fail to utilize properly in the sense that they drop the ball on either using the tool correctly, or they fuck up the impacts on the characters. As the reader, it's a skill issue because either you're taking it too seriously or maybe you missed the point of what it's in there.
Get good and maybe you wouldn't suffer so much. Thanks for coming to my ted talk.