This question came to me from over thinking the "Angulimala" story. He had to "repent" to some extent for the murders he committed, by being scolded an hit by stones. However, the effect of the karma seems to be quite mild for 999 murders and even the intention to kill the Buddha.
So my question is this: Does your karma have to be neutral or positive to become enlightened? And if not, what happens to the negative karma after attaining enlightenment?