Fantasizing about torture and murder turns me on. is this a mental illness?

Could be symptoms: Sadism involves deriving pleasure from the pain or harm of others. Attraction to such violent fantasies can occur for a variety of reasons, but a consultation with an experienced clinical psychologist or psychiatrist would be strongly advised to consider the context of such fantasies and to rule out mental illness or personality disorders.

Fantasy vs reality: There are few things that are less glamorous than most homicides -- stupidity, confusion, often-senseless despair, and two lives gone. Although many (most?) people's sexual fantasies involve dominance / control stuff, often really wild, there's a mental health issue if (and maybe only if) you feel a compulsion to hurt yourself or someone else or a serious fear you may do this. Ask someone u trust.

I can't diagnose a: Mental health disorder without an evaluation. I won't try to label you. Your behavior is disturbing. Since you enjoy this, it is unlikely that you would be willing to work on change.