Basically, Voldemort. But that's simplifying it.
Originally Posted by Mistletoe
I firmly believe that she was deeply in love with Voldemort, and if it weren't for his creepy lack of feelings thing, anyone would have to admit they're perfect for each other.
Think of it, though. When Voldy got hit with the rebounding AK that night he tried to kill Harry, she (very shortly thereafter) tortured the Longbottoms just for him. She wasn't just in love with him, she was obsessed. What did Slughorn say? "Never underestimate the power of obsessive love" (or something like that). She might have loved torture, (I'm getting to that point in a moment…) but Voldemort gave her that last little push and just made her out-and-out evil. In short, Voldy was her only power and her only weakness. Yes, at the same time. Her power because he pushed her that little bit farther into total evil, making her conscienceless and an ultimately perfect follower. But he also made her have less ambition. She wouldn't have wanted anything but to be the perfect follower. Because, honestly, she was really evil, and could easily have gotten her own band of followers if it weren't for the fact she would never have betrayed Voldemort. Weakness and strength. The whole Bella/Voldy situation is so complex. I love it.
But she just loved to torture, too, no doubt about it. I think she started out with a rather great joy of torturing. That's one of the few ways she and Voldemort differ. He doesn't see that there's anything worse than death, so he just kills with an AK, but I think Bella is more wise in that she knows there is something worse than death—she knows that being tortured is infinitely worse than just suddenly being killed. She likes to kill and torture and knows what she's doing.
But, really, nearly everything can be brought back to Voldemort. But not quite everything. I mean, she had to have been evil when she joined him, so he can't shoulder all the blame for her utter evilness/insanity.