There's an assumption there that this is the 18th incarnation of the Master (John Simm being the 17th). Given Moffat's love of timey-wimey stuff, this could very well be setting up for some future tragedy where the Doctor encounters the 'missing link' Master and does leave him for dead, thus effectively un-redeeming Missy.
Or it could be another plot by Rassilon - they captured the Master, tampered with his memory, forced a regeneration, and sent her back with another plan to free the council from their prison.
Or it could be simply what Morrus says: she's mad.
Here's a nugget of an idea. You say the Master's on Regen 18 or so. And he's evil nuts mad.
Dr. Who is now on #13, and he's been a little "off"
What if the 12 regen limit isn't a limit, but a warning. Timelords past their 12 regen shelf-life start to go sour.