Mad theory time...

[unheard_secret]

1.

Missy is short for Mistress.

Aka: the Master decided to regenerate as a woman and has embarked on some strange, possessive ego trip.

Problems with this: how is she dragging those dead souls to ‘The Promised Land’ (…which is obviously anything but).

2. 

Missy is the TARDIS gone ever so slightly mad, and she’s ‘saving’ the people who sacrifice themselves for the Doctor (in the same way CAL in the library was ‘saving’ people). 

Problems with this: Missy has been implied to be the one who wants Clara and the Doctor travelling together, but doesn’t the TARDIS dislike Clara on some fundamental level (or is that not a thing any more?)

Also… the TARDIS can’t be evil. I would cry.