Of course you can separate the art from the artist. The artist dies, the work lives on. When that happens, you can then either make up stories about their lives, or scramble those stories a bit, or get them exactly right, and no one will ever know for sure. And at that point, it makes no difference, because the work is still there, standing on its own.

Some times you can tell from reading the book that the author is genuinely evil, and that's what makes it a great book. William Burroughs was a Junky and a pedophile, and accidentally murdered his first wife. That's why reading his fiction is like staring into the jaws of hell: because he was an awful messed up person, and he had the ability to show you what it was like to be such a person. That's what made him a great artist, and makes his work worth reading.

If you found out that Dostoevsky had actually done the things he describes in Crime and Punishment, that would not stop it from being a great novel.