She was justly tried and convicted, and fully deserved to be hanged -- and worse.
The only injustice in her sentence is that her son, Bobby Lee, Jeff Davis, and all of the other traitors were not also hung and left on the gibbet for the disapprobation of the general public and the sport of carrion eaters.
So in fighting for the right to secede, Lee and Davis (et.al.) were traitors who deserved hanging?