I believe Westerners and Ukrainians are actually talking about two different things in the context of this matter (Can you enjoy~celebrate Russian literature, and at the same time condemn Russian imperialism/Russia's invasion of Ukraine? to which Ukrainians would answer No, and many Westerners Yes), so they don't simply disagree with each other but they don't even understand what the other is actually saying.
When a Westerner is like "Dostojewski is awesome, but Russia is bad" they're thinking it's a matter of two disconnected feelings. On the one hand they find a certain product of Russian culture valuable in some way, on the other hand they also think that war and torture and kidnapping children etc. is bad and Russia is bad for doing these things. It's not a contradiction for them, it's just not mutually exclusive, so they don't see the problem.
I do actually understand their perspective if you look at it on such a superficial level, without any other, deeper considerations (which Westerners have the luxury of doing, Ukrainians don't). Yes, a certain poem or whatever, by a Russian author, can be well-written and smart or deep etc. Of course not every single thing that a Russian brain comes up with is automatically bad in every way because it was a Russian who made it. I have encountered a few Russian songs for example of which I think "damn, that's actually a good song" (and I don't listen to them anyway, or don't even mention it, reasons below). And that's where the whole line of argument ends for Westerners, so they find Ukrainians crazy for denying this. But actually they don't.
Actually it's even the opposite. Ukrainians know much more about Russian culture, and have been exposed to more and in deeper ways to products of it, than any Westerner. They know many more good Russian books (actually good or just the kind that they used to like for some reason or another) or plays or songs than Westerners, and they might even have built [positive, originally] emotional relationships with some of those because they grew up with them and nostalgia is a bitch.
This is just not the point. The point is...
1) You can't separate the art from the artist because the art is literally an external manifestation of the artist. That's what art is. If you can't separate the art from the artist, then celebrating the art makes you by extension celebrate the artist. And you can't change that by contradicting comments, like saying that you don't see it that way so it's fine. If I tell you I can fly so it's no problem to jump off that bridge, that doesn't change the fact that I'll die if I'd do it, and every reasonable person knows that, no matter what I proclaim why I'm special.
And fact is that the vast majority of Russian artists very much shared/share exactly those parts of Russia's culture that made this war happen and keeps it going. So by validating them you automatically validate what they stood or are standing for, imperialism, fascism, 'Russian superiority' etc.
2) Another important factor here is a practical one. Regardless of who this Russian artist is and what you're thinking about them, you do support Russia one way or another by consuming or commenting on Russian culture, no matter if that's intentional or 'accidental' (can't really be accidental anymore at this point where everybody should be aware of this fact, but anyway). Be it directly by paying money to buy or see this piece of Russian culture and possibly encouraging others to do the same, or indirectly by advocating for Russia.
And you do inherently do that by defending Russian art, whether you want to or not, pragmatically speaking, that's the real world effect. Russia is terrorizing Europe for centuries and currently trying to annihilate a whole nation. If you choose to focus in your thinking or communication on its 'positive sides', or what you're judging as those, you're actively setting a 'counter-weight' to its crimes and atrocities and genocidal attitudes. Like... okay on the one hand they're fascists who torture kids, but on the other hand they're also fine writers. This kind of argument just automatically gives off the impression that one thing would be a defense for the other. As if you could weigh these things against one another (and possibly end up in a state of balance, so it's actually okay?!) It's not and you can't, and it doesn't matter if you personally agree with it, that's a message that you send. You're supporting Russian propaganda this way, giving certain desirable (by Russia) aspects more room in the public discourse while distracting from other undesirable ones, which are the ones that matter much more.
If you're actually on the side of the victims, you don't want to do that. And you choose to use your limited time and energy on advocating for them, not for the aggressor. There's great Ukrainian art too, you know...