It's obvious that Bill Clinton went on the show fed up with the ongoing smear campaign against him and the whitewash of the current administration, and Clinton was prepared to fight back. As one commenter on Think Progress put it, "It’s about time we started standing up to the Rightwing in solid ways. With the truth, not by playing their game. It takes the Big Dog to do it. Maybe others will catch on."
In his memoir My Life, Clinton expresses his exasperation with Democrats for not fighting back. He has been smeared more than possibly any other public figure, but he fought back - on the facts - and he fought back hard, and it worked.
A talking head on CNN opined this morning that Clinton had no reason to complain about the questions the Fox interviewer lobbied at him; she said he should have known that he would get that kind of question on Fox. She got the entire situation backwards: after smearing Clinton for years, Fox should have known that it would be very dangerous to give him a chance to respond to their viewers. I wouldn't be surprised if someone at Fox News isn't quietly fired over this.
By the way, I also saw Clinton today on the much more respectable Meet the Press, and he talked in elegant detail about the $7 billion he just raised for his international aid foundation - the ostensible reason he was on Fox News. Fox was just one station on a long line of media stops for Clinton to publicize his foundation.