I realize this might not be the best place to hash this out due to the format but I feel compelled to respond. I just encourage you personally to think about this matter holistically. That is, people dont do, say, and think things in a vacuum. They _inherit_ their notions and behaviors from their environment, and that's all I meant by "ideology." You can rest assured that when I use the word ideology I use it advisedly and after much thought.
No matter what you believe, or what he believed, evil is only ever done by human hands and in this case those hands were moved by a world view and belief system that were outlined in a (absurd) 78+ page "manifesto." We can not afford to ignore this fact. Nor can we afford to ignore that as with any line of thinking about the world there are many other people who have adjacent and related beliefs. Yes, it might be the case that what makes people who think that way into killers is "nihilistic sociopath[y]" but the unfeeling among them are raised and encouraged by a whole community. That's the real reason why I think it's important to look past the incident in isolation and remember:
This man has a community.
We can't undo what happened but we can each individually make an effort to understand why people end up in that community, how to identify them, and hopefully how to keep our sons from ending up there in order to keep another such atrocity from happening again. Part of that is, as you say, by "wholesale-reject that outrageous, inhumane way of thinking". It just seems clear to me that it must be actively rejected. We must actively deplatform and remove people who parrot what he did from our community. To that end we must understand why it is and has been for hundreds of years so alluring to people that it leads a subset of them to some of the worst violence humanity perpetrates.
The oppressed can never afford to ignore the beliefs and, yes, ideologies of their oppressors.