The fetish does nothing by itself, action does. Preaching a religion that advocates murder is legal and all, practicing and putting it into action is when the law gets involved.
Until you develop some Psyco Pass tech that can catch people before they will undoubtedly commit a crime, there is nothing to be done without creating thought crime.
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As law abiding citizens they would need to consent to the rehab. You can't force it on an unwilling person. The required consent doesn't change or act differently because of a fetish, sorry.