Why?
He told the truth about Hillary. He probably has the entire Encyclopedic set of case files on the bitch...ready to deliver to Sessions.
Comey made it sound like he wanted to prosecute Hillary and had ample evidence to, but had been directed not to. Might be smart to retain the guy.
No he didn’t, he contorted all sorts of lies by which he concluded she and her staff had no intent to do anything wrong. He also made sure that key players weren’t questioned under oath, had time and means to coordinate stories, didn’t hand over critical evidence, etc., etc.
Nothing Comey could half-heartedly say or do could make up for his obstruction of justice in a case by which Hillary should already be “locked up”.
That and he can be brought up on charges for the way he handled the whole matter...compromising witness testimony, burying evidence, etc. He’s the foremost authority on the matter and knows where the political battle lines are drawn.