I haven't seen much to suggest he personally did things that would warrant a criminal investigation - but obviously there are such suspicions about Flynn, and at least a serious possibility that it goes well beyond him. Firing the person in charge of handling that, in a period when he is testifying about it, subpoenas are being sent out, etc., was always going to make it look like Trump has something to hide. If there are other good reasons why Comey needed to be fired as soon as possible, rather than in due time at a less controversial moment, maybe the additional suspicions could be a price you're willing to pay, but I'm not really aware of any. Certainly Trump's letter only made things look worse, with its blatantly unbelievable motivation for the firing.

Which rather reinforces the credibility of the various leaks which have confirmed the suspicion that this had at least something to do with the Russia probe. Whether that 'something' was about Trump protecting himself or merely protecting someone in his staff, that's another question.

I guess, as stated in the article I shared with Tom below, that the conclusions to be drawn on this will depend on who Trump tries to nominate as new FBI director. If it's a serious candidate with bipartisan support, that would make the firing look less bad in retrospect, because it would go a long way in refuting the suspicion that Trump's motivation was just to make the Russia probe go away.