Longstreet did what was best for him after the war was over. Many of those who subscribed to dignity in defeat and poverty didn't cotton to someone who wasn't down for martyrdom.
Longstreet has been cited by a couple of sources as the best tactician on either side of the war, and I'll definitely say he was a power once the battle started, unless, as at Gettysburg, he was acting petulant.
Longstreet was loyal, and stalwart to the very end. To we who criticize his actions after the war as a betrayal, I would remind of his comment to Lee, when Grant insisted on surrender initially: "Not yet.", and his lambasting of that punk Custer when he had the audacity to ride into the rebel lines under a white flag and start making demands.
One of the best generals, but, as Lee said........"so slow".