Dear Sir, i am just an ignorant girl, but if i may speak, i name sadist a person who finds sexual pleasure in torturing or humiliating other persons. If somebody is cruel, it doesn't mean that he/she is a sadist.
My sweet Mistress is a sadist, no doubt. She gets aroused by whipping me to bleed or making me lick up her spit. At the same time she is a tender lover.