Objectively he committed the crime, and he also had a very negative attitude after the war which did nothing to help matters.
But Davis wasn't really worth the trouble, and so it was dropped, and I agree with that.
Lee, on the other hand, did everything he could to help rebuild the country.
Trying and executing Lee would have been considered an outrage in the North as well as the South.
It would not only have been unjust, but also extremely stupid.
Really, DAvis ASKED for a trial. No one in Justice wanted to try him because that would put secession on trial and they thought they would LOSE.