The Master is the Doctor's archenemy, and a renegade Time Lord to boot. He and the Doctor were childhood friends on their home planet, Gallifrey, until they went their separate ways. The Doctor went to explore the wonders of the universe, while the Master went to conquer it instead.
The two Time Lords share a rich and complex history together, with both of them unable to kill the other. Generally, the Doctor always wins in the end, but the Master has come up with some truly evil and audacious schemes. As flawed as he is, unfortunately his character has gotten worse over the course of Doctor Who — and not just in a villainous way.
Disregard For Human Life
It's possible that the Master hates humanity because they act as a rival for the Doctor's affections. Certainly, the Doctor appears to love visiting Earth more than any other planet in the universe. Or, it could just be that the Master views the human race as insects.
Whatever the reason, the Master has never shied away from taking over the world and killing a few million humans along the way. He knows that Earth is important to the Doctor, and takes great delight in disrupting the Doctor's haven.
Control Over The Cybermen
The Cybermen are "upgraded" versions of the human race, born out of a desperate need to survive. Therefore, they usually have agency over their own thoughts and actions. That is, until the Master wanders in and simply takes control of entire armies of Cybermen.
Since Season 8, there have been three season finales that feature the Master doing this. Each story is different, which is good, but the fact remains that the Cybermen don't need the Master, and yet they bow down to him whenever he appears.
Feud With The Doctor
The Doctor and the Master have been engaged in a seemingly endless battle for what seems like forever. The first locked horns during the 1971 serial, "Terror Of The Autons," and they haven't stopped fighting since.
Even in the modern series, they're still at it. What makes this time worse is that when the Master first returned in 2007, he and the Doctor were the last two Time Lords left in the universe. One would have thought they'd stop fighting then and finally reconcile. Unfortunately, the Master soon put a stop to that silly notion.
Betraying The Doctor
When the Master regenerated into his first female incarnation, Missy, it led to one of the more complex versions of the Doctor's enemy to date. Missy was still delightfully evil, and reveled in being able to kill the Doctor's friends, but there was something else too.
Missy didn't want to kill the Doctor. Instead, she wanted them to finally be friends again, and did everything she could to accomplish this goal. Ultimately, she decided to be good, and it was even starting to work. However, when her old self turned up, Missy switched sides and betrayed the Doctor.
Retconning His And The Doctor's Friendship
The Master's most recent appearance in Doctor Who was in Season 12. He had a new face and was probably the character's angriest incarnation to date. However, there was one slight problem: the last fans had seen of the Master was her redemption.
Missy had been going back to help the Doctor fight the Cybermen when she had been killed. Therefore, this new Master should be feeling warmer towards his enemy. No such luck. If anything, this version of the Master hated the Doctor more passionately than ever.
Turning Bill Into A Cyberman
Bill Potts was one of the Doctor's shortest-serving companions as she was only in the series for a mere 13 episodes and hasn't been back since. That being said, Bill was incredibly popular and was a massive hit with fans and critics alike.
The Doctor also loved her, so naturally, the Master had to find a way to take Bill away from him. It was incredibly cruel as the Master ultimately killed her with kindness. He pretended to be her friend for years and then sold her out to the Cybermen to be upgraded.
Toying With The Doctor's Companions
In the old series, the Master was more obsessed with getting straightforward revenge on his nemesis, the Doctor. However, in the new series, it appears that he has developed a crueler edge. Missy, in particular, took great delight in tormenting Clara Oswald for two seasons.
In Season 8, she actually brought Clara and the Doctor together, knowing that that combination would eventually result in the Doctor investigating the afterlife. Then, in the following season, Clara became Missy's pseudo-companion, which resulted in Missy leaving Clara to the mercy of the Daleks.
Inability To Be Killed
The Daleks are Doctor Who's most iconic monster, universally loved by fans of the show. Therefore, they've made the most appearances of any villain. However, that means that by default, they are also the most defeated monster in Doctor Who history.
The Master, unfortunately, also suffers from this affliction. It becomes increasingly difficult to make the Master seem like a constant, viable threat when he's been thwarted by the Doctor so many times in the past. Nevertheless, the character remains ever popular.
No Explanation For His Returns
One of the most annoying things about the Master is that he always returns from the dead. Believe it or not, this trend actually started in the 1970s, with the Master being defeated in one episode, and coming back in full force the next, with no explanation being provided as to how.
Recently, the modern series of Doctor Who has adopted this trend, with Missy trapped with the Daleks in Season 9, only to appear in Season 10 about to be executed. Similarly, Missy seemingly died for good, only for a new Master to appear two seasons later, again with no explanation.
Interfering With The Belief System
In Season 8, Doctor Who portrayed one of its darkest and probably most controversial storylines ever in exploring the afterlife, known as the Promised Land. However, what could have been a fascinating philosophical direction for the show ended up being just one of the Master's more audacious schemes. Missy uploaded dying human minds and held them until they could be downloaded into their Cybermen bodies.