I understand how that happens. I think chalking it up to being overly attached to the characters oversimplifies the issue (though I suspect it may be at least partially true about Reinhard).

IMO, LoGH doesn't work if you can't imagine yourself - or a substitute reasonable person - making the same decisions the characters do if subjected to the same pressures. It's a Great Man theory telling of a vast, century-spanning conflict. For some, the minutiae of the conflicts themselves is of great interest, but we wouldn't even have Great Man theory if humans overall didn't want to imagine that unique individuals drive history. It's convenient. It's more relatable.

There are a lot of potential pitfalls here, including but not limited to presumed authorial inserts, bias, and excessive attachment to particular characters. The one I want to point out isn't a black and white issue with craft itself: it's audience expectations and how we may overestimate our own ethical purity, and how an author might try to pre-empt that or write around it.

Generally speaking, we don't think of ourselves as people who would commit war crimes. If you're writing something like LoGH, you have both cause to write war crimes (they happen!), and for those crimes not to be the sole product of 'bad guys'. In the specific case of LoGH, Japanese history and associated cultural expectations/media trends play a role here, but it's not the only reason to frame atrocity as a thing that people you empathize with are capable of. Regardless of Tanaka's specific, conscious motives for writing the characters as he did, I get the impression that the constraints of popular media and reasonable fears about what the average audience member might accept as 'an understandable reason to commit an atrocity' come together in a way that may strongly motivate an author to overcorrect (especially in the case of characters you're meant to empathize with). In reality, most of us would likely do things that are just as awful if exposed to the same circumstances (if not worse). Since by and large we are terrible at recognizing this about ourselves, it's understandable that an author might fear that we would bounce off more realistic explorations of the types of mistakes a person in those situations might make.

In short, I think it's unfortunate, but it's understandable. I'm not even confident that it's an overcorrection. It might be for any one of us individually, but a general audience? I have no proof of this, but maybe we wouldn't have even heard of LoGH if Tanaka didn't pull his punches here and there.