Speaking strictly as a retired federal law enforcement officer and supervisor, a president who was not ethically compromised and did not have a partisan act to grind would indeed have plenty of cause to fire Comey over his handling of the Clinton e-mail investigation but not for the reason Clinton supporters think. The original case should have been presented to a DOJ prosecutor last June, who then should have presented it to a grand jury for deliberations as to whether an indictment was warranted. Comey had no business acting as both investigator and prosecutor when there was indeed evidence that Clinton broke the law.
Whether it was his intention to do so or not, Comey covered for Clinton knowing that left to its own devices a grand jury probably would have indicted her, which would have been the end of her candidacy. Ironically, he also placed himself in the awkward position of having to notify Congress when additional evidence was uncovered, even though when such evidence was uncovered it ultimately did not change his initial (wrong) decision. Clinton has blamed Comey’s October letter for costing her the presidency, but that letter was a consequence of Comey’s earlier action that protected her.
When I was still working and we conducted investigations involving senior officials, we referred every case to a DOJ prosecutor for consideration of whether presentation to a grand jury was warranted. By doing so we immunized ourselves from any charges that we were biased in favor of the subject of the investigation or were part of a coverup. Once Comey made the unilateral decision not to refer the Clinton case to a DOJ prosecutor he compromised not only his own integrity but that of the FBI. For that reason alone he should have been fired, even if Trump is the wrong person to do it.