Cassandra's Back-story
This post contains spoilers for pretty much all DC comics canon in which Batgirl participated. Especially the eponymous run of Batgirl.

Cassandra Cain is the daughter of master assassin David Cain and Shiva Woo'san (who is widely regarded to be the most deadly woman alive). She was left in Cain's care while Shiva continued her own training.

Cain had some very interesting ideas about training people. What he wanted was the perfect human weapon, and he thought the best way to achieve that was to train someone to read body language fluently. He realized that in order to do that, he would need to begin training before the language centers of the brain could stabilize, and that meant he needed a child.

Cassandra wasn't Cain's first attempt, though she was the first (and only) child to both survive the training and remain (relatively) sane. Cain's methods of teaching were pretty brutal. He raised Cassandra in an environment where no one every talked to her, forcing her to recognize movements as a form of communication. He constantly tested her against various opponents.

Further, in order to "keep her on her toes" and help her develop her situational awareness, Cain would shoot Cassandra randomly. He also used guns to increase Cassandra's tolerance for pain by playing a variation of "two for flinching".

Eventually, Cain decided that the time had come to test Cassandra for real, and at the age of eight he sent her on her first assassination. This ended up backfiring. Cassandra located her target, took out his bodyguards, and then ripped out his throat, but... She saw him die, and that meant that she understood exactly what she had done.

So she ran, fleeing from Cain and unable to forget the horror of what she had done.

There's more, which I may one day actually add, but this is the stuff that shaped Cassandra most strongly.