Sadism is the most evil thing a human can indulge in.   stance: yes:
Sadism as the practice of violence or cruelty against others to fulfil some sort of personal or sexual gratification is, in my opinion, the most sickening act a person can commit. In order to commit such an act, the person must be subjugated and therefore weaker than the active participant, thus compounding the act of violence. This is "evil".
However where both, or all individuals are consenting active participants, I believe this to be a form of masochism, albeit from the hands of another. Although this statement may seem to be an oxymoron, if you pursue pain and violence, and consequently receive it from the hands of another, is this not just "assisted" masochism? (a la assisted suicide) Although these individuals may be strange, what two (or more) consenting adults do in private is no concern of mine.
Pick this apart at your will.