Also, last time the Doctor tried to get into the TARDIS, his key didn't work. I suspect that the Master let him in, to let him see what had been done to the TARDIS, as a part of his punishment of the Doctor. That is a horrible thought as well.

Going along with the thought of possibly wanting to punish the Doctor, others have thought that the Master may use Rose to get to the Doctor as well. Of course, this scenario is probably not going to happen but it would definitely hurt the Doctor.