I think a lot of it stemmed from her family and childhood. A pureblood family would most likely want a son so that he could inherit the family name, titles etc. but instead they got 3 daughters (I'm guessing it's like this) so they would have gotten some disapointment from their parents for this.
So if from her birth she was already a disapointment to her parents she would have tried her utmost from then on to get their approval and pride. One method of doing this would be to open herself up completely to te pureblood mania that her family were famed for.
I think also she would have tried to become the male heir that her parents craved, she probably became strong, independant and powerfull in her mid-teens by immersing herself in the dark arts. She then later joined the deatheaters, a group that is mainly made up of men, which also suggests that she is trying to prove herself just as good as a man.
She married Rodolfus to please her parents by making a good pure-blood marriage, but he proabably didn't respect her so she sould try and get his approval and respect by having an aptitude for torture, murder and other such deatheater activity.
Because she was so devoted to, an had so many reasons, to get involved with the torturing and murdering she would have developed a need and obsession for it and so grew to thrive on it. It was her method for survival, her way of living.
Because Voldemort was the only one who actually was impressed with her natural ability as a deatheater (Rodolfus probably wasn't too happy when she outstripped him as a deatheater as I assume she did)she took her obsession to please to him, as she had found a master who would reward and appreciate her, so she would do anything for him.

That is my theory anyway. I wasn't planning on writting that much but there you go.