The Republic hero known as “Revan” had a turbulent past. As a Jedi general in the Mandalorian Wars, he led Republic forces to victory; as Darth Revan, corrupted by the will of the Sith Emperor, he became the Republic's deadliest enemy. Betrayed by his apprentice, Malak, Revan lost his memory and was given a new identity by the Jedi Council; he then pursued and destroyed Malak, redeeming himself before his memories of the Sith Empire began to return.
Revan left known space intending to find and stop the Sith Emperor, but his plan failed. He was captured and held in stasis by the Emperor for more than three hundred years, where he suffered under the Emperor's mental interrogation. He escaped at last, and it is clear that centuries spent linked to the Emperor's mind had a profound effect on him. During the Mandalorian Wars, Revan was willing to sacrifice his own troops to achieve military objectives; his creation of extermination droids now goes even further, demonstrating Revan's fanatical belief that any sacrifice is worthwhile to stop the Emperor.
Perhaps he is mad‒the cost, paid willingly, to battle the Emperor's will and even exert his own influence on the Emperor to keep him in check. Or perhaps the ideals of the Jedi proved insufficient, in the end, for a man neither Jedi nor Sith.