Shizuka was a little crazy herself. She wanted to die, that's the only thing I can think of. Purebloods can't just commit suicide though. They don't die.
[Thinking good things about Shizuka, was not easy for him. But there were things he couldn't discount. He wasn't one for spilling his personal story, but more than anything he wanted Buffy to understand where his allegiance sat and why. That he'd chosen to continue to classify himself as a hunter, because that's where he belonged and where his goals sat.]
The how and why for everything is long and complicated, but Shizuka was engaged to Rido when she fell in love with a former human. My parents, through what many consider to be conspiracy, were selected to hunt that vampire, but he wasn't Level E yet. What she did to my family could be considered an act of revenge, but Ichiru always romanticized it and claimed it wasn't.
After losing the one she loved, she did give her blood to others even though it was forbidden. There were requirements though. They had to help her with something important and she had to care about them in some way.
Ichiru assisted her that day, left with her. For four years I thought he was dead. He claims she took care of him, gave her blood to him to cure his illness. She never turned him, never intended on turning him. She also borrowed the body of one of her relatives to get around in, in return for her blood.
The only real reason I can come up with for not doing it right away is because she wanted me to earn it. Knowing you're not going to be sane for long sort of gives a sense of urgency to the whole 'revenge' aspect. She wanted that hatred from me.