Geralt is a huge softie. He's constantly driven to help others even when he knows it's not worth it. The Butcher of Blaviken nickname is just a misunderstanding. In the end, he
Spoiler: Major book spoilers, but you know this if you played the games.
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dies trying to protect nonhumans from oppression.
That's why The Witcher never
felt like real dark fantasy to me. It's full of sympathetic characters and humor, and usually aims to be bittersweet rather than depressing. However, since the definition of dark fantasy is loose, one could call TW that and it would be pretty hard to refute.