Well, the reason that Angel left Sunnydale and Buffy, aside from the fact that he seemed to fear losing control of himself and, with Buffy's eager assistance, lose his soul for a second time, was so that he could somehow atone for all of the pain and death that he had inflicted over the past couple of hundred years. As far as becoming human, that possibility didn't even enter into the mix until he accidentally discovered the Shanshu Prophecy scroll some time later. In fact, he even went so far as to reject humanity when he regained it in "I Will Remember You."
I don't really think, though, that, from that point on, it is ever really made clear exactly whether his ultimate goal was to attempt to somehow balance or even tip the scales of his life back to the positive side, or if he was only doing that in hopes of being rewarded for his actions by becoming human once more. Hopefully the former.