"A saner man would have killed me by now." Yep, you've summed it up admirably! A deliciously dark story!
Lovely mix of smut, angst and banter. My three favourite things in fics :)
You've got the Master's dialogue really spot on - I can hear him saying these things to the Doctor. With all his usual blend of sarcasm, anger, honesty and madness.
Plausible based on established.