Anarchists usually think that a lot of murderers actually get away with it in our actual world, be it through war crimes, neglecting sanitary or safety rules to maximise profit; you can extend this list with a lot of legal murders.

Anarchism definitely does not define a specific rule for what to do with murderers. Different communities might want to handle that differently. They usually think that prison does not solve anything though, and that only the poor get sent there anyway.

I think a mistake is to think that anarchism is a "feature-complete" view of the world, when it really is the realisation that power corrupts, and that we should keep this in mind when organising ourselves. Arguably, over the long run, anarchist views are winning: institutions that prevent - in theory - crazy psychopath from taking absolute power, churches losing power over our lives, women considered as human beings; these are things anarchists have pushed for, for 2 centuries. This short essay might give you more insight: https://theanarchistlibrary.org/library/david-graeber-are-you-an-anarchist-the-answer-may-surprise-you