For me it all comes down to timing and questionable optics.
Do I think Comey deserved to be fired? Probably so.
But the timing of the firing needs to take into account how things might appear in terms of conflicts of interest, personal culpability, etc.
Had POTUS fired Comey and at the same time called for a special prosecutor that would report only to the Deputy AG (to avoid the conflicts of interest with the AG, as with the existing investigation), then I would have had a different reaction. That would have sent the message that he was taking the investigation seriously and did not want anyone to think that he was making his decision with Comey with any ulterior motives in mind.
I would be 100% supportive, if he did that.
I'll even retract some of my criticism if he calls for a special prosecutor even at this point because that would show very, very clearly that he's not trying to hide anything. And if he is innocent, he has nothing to fear. By calling for an independent prosecutor himself, that would show confidence and integrity. He'd come out looking pretty good, unless there really is something he's trying to hide.