Johnson was a long time Democrat who turned out to be a lot softer on the South than most Republicans were. His idea may have been that freed slaves would vote with poorer whites to offset the power of the planter elite. In any case, we did eventually extend the suffrage to people of different races. Was that something we shouldn’t have done?
My rule of thumb is only tax paying citizens should vote, but that's not the main point here. The liberals created this right for them simply to gain power, not because it was the moral thing to do.
The mens rea was evil, just as it was for their reasons for starting the Civil War. They did these things for selfish reasons, and then pretended they did them for moral reasons.
For what it's worth, in the various readings I have done on the issue of the Civil War, I had come across an article alleging that this (enfranchisement of freed slaves) was the specific thing that motivated John Wilkes Booth to assassinate Lincoln. Perhaps others on the forum know more about this, but I have ran across that claim.