Yet, by installing Military (martial) governments even in "loyal" states, that's precisely what he accomplished. Legislators, newspaper editors, and prominent citizens that thought he was overstepping his lawful bounds in his prosecution of the war were thrown in jail and denied the right of trial. Is that the "lawful" way to amend constitutions, enact legislation, and "ensure a Republican form of government to each State?"
As an example, the Federal government orchestrated the overthrow of the lawfully-elected Government of Virginia, stationed troops outside of the polls in our Western counties (which, admittedly, were not in very close agreement with the Eastern ones already) in order to intimidate "non-loyal" voters, and allowed a puppet government to be stationed in Alexandria, under the farce of a notion that the "Government of Virginia has been vacated." (In blatant disregard of the fact that the lawfully elected legislators were still in their Seats, in Richmond.) This puppet government, staffed entirely by people from Western counties, then proceeded to vote to split the Commonwealth in two, essentially stealing a vast portion of land from a sovereign nation. (When the war had closed, Johnson issued a proclamation declaring that the farce of a government in Alexandria was to become the new lawful Government of Virginia, thus completing the takeover that had been begun in 1861.)
"Dictator" may not be the exact word I'd use to describe him, but I wouldn't exactly think of him in the kindest terms... He and the radical Republicans in Congress didn't exactly follow the letter nor the law of the Constitution, by any stretch of the imagination.
I would, since he broke the law he swore to uphold, then changed our form of government by his actions.