It is possible to accept that things happen that you don’t endorse or control, horrible things, without providing a justifying reason for those events.

Unless, of course, you are the one knowingly carrying out those acts. In that case, a level of nihilistic justification is necessary from both leadership and those doing the dirty work.

Death, vile and evil behavior are a part of life. This doesn’t justify their existence.

Nihilism promotes the idea that life is meaningless. Perhaps an attractive idea in a complicated world with countless examples of suffering, horror, and death. I would urge you to consider an alternative conclusion when faced with these realities: The presence of death is necessary for life itself to have any distinct meaning. Once you accept that death is a necessary element for any “life” framework ever evidenced or argued for, vile behavior like systematic genocide can be understood as an outgrowth of the intertwined nature of life and death.

This still doesn’t justify a nihilistic conclusion. For example, if I dropped my cup and it broke, I might be tempted to provide a reason for the destruction of that cup (I was distracted, the coffee was hot, etc.). These explanations might be appropriate, or they might not. Since we are examining the event after it happened, we won’t ever know the exact causal reason (though we might get really close!).

The post-hoc rationalization of past events can only go so far. The causal reasons cannot be fully sussed after an event. (You can get pretty darn close! So it’s worth rationalizing explanations for the possibility of prevention of something like that.)

Regardless, these explanations are not necessary to accept the reality of my broken cup. I may dearly want to know why my cup broke. Perhaps because I liked that cup, or because I simply don’t want to break another cup in the same way, but knowing the reason my cup broke is not a necessary or sufficient element for me to know that I miss my cup and, all things considered, would have rather not had it break.

In the same way, the suffering you see in the world does not need to be justified for it to be condemned.

So, because nihilistic beliefs are (a) unnecessary for condemnation of suffering and evil (bad things are bad, A = A), (b) highly useful for justifying suffering and evil (as you noted), and (c) naturally lead to (at best) inaction in the face of suffering and (at worst) acceptance or even endorsement of suffering (“Well, life has no meaning, so I may as well accept the suffering, and maybe even go so far as to fuel a bit of it for my own benefit. After all, there isn’t a meaningful moral difference between those two options.”)

With those three outcomes as just a few of the natural consequences of practicing nihilism, I fail to see either the logic or utility of nihilism in anything other than providing a very shallow justification for the suffering in the world. It’s an easy puddle to splash around in, but I would encourage you to keep thinking through it: You don’t need to reject morality and ethics simply because immorality, unethical behavior, and unethical people exist.

As a final note: If you want to keep your nihilistic beliefs, you must reject the stated premise above that bad things are bad (A = A in symbolic logic). However, I’d suggest you be very careful about rejecting what you feel to be true (i.e., that destructive and horrible events are indeed destructive and horrible).

Hope this helps. Try not to get trapped by nihilism. It doesn’t lead to anything meaningful, as it’s logically impossible to do so under that framework.

I think the key here is "mistake". I don't really like that word here, any more than I like "precious" as opposed to interesting. Many rare things are just dangerously destructive.