Doctor Who: 10 Worst Things The Master Has Ever Done
2. Destroying Gallifrey
Creating a near-unstoppable race of regenerating Cybermen was definitely bad, but that's nothing compared to what the Master did to get there.
After learning the truth about the Timeless Child and discovering that a small piece of the Doctor lived within him, the Master - in a fit of rage - absolutely ravaged his home planet of Gallifrey, murdering untold amounts of Time Lords in the process.
While the Master is obviously known for causing death and destruction, it was still really shocking that he chose to strike a blow like this, in a place that's so personal to him - and to the Doctor. It's understandable why he's angry, but actually having the guts to annihilate his own people really demonstrates how cold and uncompromising the Master has the potential to be.
This incident is also a fine demonstration of the Master's strength. Basically no other individual in the universe would be capable of a feat this destructive, and while it's still unclear how he pulled it off, Gallifrey - one of the most powerful civilisations out there - is just gone, and the Master is the one responsible for that.