I'm going to vote for the neural parasites from TNG Conspiracy. I mean, there's an argument to be made that they, like any other parasite shown using or feeding on creatures, are just following a natural design, that requires they exact this atrocity on whole species, but not only is that less certain as it is with other victimizers we've seen, they also seemed especially thrilled, particularly to the point of revelling in the fact that they were subjugating and enslaving beings, down to their very core.
They weren't just hunting, consuming, destroying, torturing or otherwise using people. They were choosing to deliberately do maybe all of that, for a seemingly less understood & possibly less necessary reason, & really seemed to enjoy that they were. It's a pretty nasty business, what they were up to, & despite being intelligent, they not only gave zero f**ks, they were actually cruelly loving it