The Master's sixth incarnation regenerated after losing too much life force. After his regeneration had completed, he instantly ran away from the Doctor. After a lengthly chase, the Master stole a car and drove away. The Doctor continued to pursue him, but to no avail. When the Doctor finally tracked him down, their discussion was interrupted by the Dream Lord.
This Master was definitely not insane. However, despite regaining his sanity, he is now more evil then ever before. This Master took pleasure in manipulating both his allies and enemies to achieve his goals. As well as being the most evil incarnation, he also had the most complex personality.