WordNet (r) 3.0 (2006):

sadist
    n 1: someone who obtains pleasure from inflicting pain or others
         [ant: masochist]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "sadist":
   annoyer, badgerer, bugger, bully, coprophiliac, erotomaniac,
   exhibitionist, fetishist, harasser, harrier, heckler, masochist,
   nag, narcissist, necrophiliac, nudnik, nudzh, nymphomaniac,
   paraphiliac, pederast, pedophiliac, persecutor, pest, pesterer,
   plaguer, rapist, satyr, scotophiliac, sodomist, sodomite, tease,
   teaser, torment, tormentor, transvestite, voyeur, zoophiliac