I would tend to agree. Of all the many dumb things done in the name of secession and Southern independence, the murder of Lincoln and the attempted assassination of Seward were probably the stupidest. I can understand them as the flailing acts of a few men in a dying cause, but the harm they did the South far outweighed any fantasy of vengenance and retribution the plotters may have had.
I also agree that Reconstruction was fairly lenient by the standards of the day. The hundred years or so of history before that show plenty of examples far worse than Reconstruction ever was: "The Terror" of Robespierre and the French Revolution, the repression of uprisings in Spain and the Tyrol and La Vendee by Napoleon, the aftermath of the Revolutions of 1832 and 1848 in Europe, etc. -- if those aren't bad enough, we move on to the world outside mainstream Europe, to the Balkans and India after the Sepoy Mutiny and Russia and ...
If we imagine Lincoln presiding over those first 3 years instead of Adrew Johnson and the Radicals, then handing off to Grant and perhaps staying around to advise him, well ... things would certainly have been better for the South.
ADDED LATER: Imagine the chaos that would have arisen if they got Vice President Andrew Johnson as well. That would have made
Lafayette S. Foster of Connecticut, President
Pro Tempore of the Senate into the President in 1865.
Tim