I'm not so sure about this distinction. For most people, being a hedonist means enjoying food and fine wine and comfort etc. But if the thing you're deriving pleasure from is killing people (or torturing them with railroad spikes)... you're a sadist, and not in a nice consensual-BDSM-sex kind of way.

Saying that Spike was a hedonist is as true as saying that Angelus was an artist because he enjoyed the artistry of torturing people.