Doctor Who: 10 Worst Things The Twelfth Doctor Ever Did
4. Abandoning Ashildr In The Girl Who Died
The Doctor more than most should know immortality can be more of a curse than a blessing, so it's all the more puzzling why he'd subject that fate to someone else.
When the Doctor and Clara travel to a Viking village they encounter Ashildr, a farmer's daughter who helps the to save her village from the control of an alien species called Mire. Unfortunately, while the Mire were fended off, Ashildr was killed in the process.
Feeling guilty over her death and the countless other people the Doctor has lost, he decides to use Mire technology to revive her, but in doing so makes her immortal. Later in the episode when talking to Clara, he even confesses that he acted rashly and may have made a terrible mistake.
What's worse is that he abandons Ashildr, meaning she is completely unprepared for what she's about to experience. When the Doctor encounters her again he discovers she's lived a lifetime of pain and loss.
While the Doctor was trying to save Ashildr, his rashness lead to all the suffering she's had to endure. If he wanted to save her, he could have talked to her before leaving and tried to prepare her for what she was going to experience.