If you watch series 8 carefully, there is a clear story arc. The story arc is the Doctor's character development. The Doctor's appearance is old: remember what Madame Vastra said, that the Doctor has lifted his veil and revealed himself, changing from young to old. Parallel to that, his character is more open-book type, unlike his predecessor (the 11th Doctor) who always hid his feelings and made himself a closed book.
The 12th is beginning to question himself in the series (and in series 9 too):
- Starting in Deep Breath, in his confrontation with the mechanic robot.
- Then in Into the Dalek, where he questions Clara "Am I a good man?", to realising that he is a "Good Dalek" and full of hatred.
- In Robot of Sherwood, he states that he is not a hero to Robin Hood, indicating that he is still questioning his identity.
- In Listen, we explore the Doctor's fear and how he overcomes it. We can also deduce from this episode that the Doctor is always in fear because he always runs and is brave.
- In Time Heist, the Doctor hates "the architect" upon realising his identity. He is quite hating himself, though I'm not really sure how to elaborate this one.
- In The Caretaker, the Doctor expresses his care for his companions, shown by his early disapproval of Clara's dating Danny because he was a soldier. But then, as long as his companion is safe, he is okay with it.
- In Kill the Moon and Mummy on the Orient Express, I'm not really sure about it, so I will skip this one, but I think the Doctor is dealing with his attitude and relationship with Clara.
- Contrary to Kill the Moon (maybe because of his tense with Clara in Kill the Moon), In the Forest of the Night shows the Doctor trying to help to save the Earth, and not letting humanity act alone (this development leads to the part of his speech "passing through, helping out").
- In the series finale, we see the Doctor is helping Clara to find Danny in "hell" even though that might break the laws of time and also even though he has been betrayed by Clara. That is because he is trying to be the Doctor, always helping no matter what. And in the end, he is confronted by Missy, who wants to help him save the universe. The Doctor, who always wants to save the universe, is faced with this moral act of using the cyber-bracelet.
He is remembering his promise of the Doctor: "never be cruel, never be cowardly". Using an army to defeat monsters is a cruel and cowardly thing, one thing that the Doctor doesn't want to be. So be it, he is realising his own character: full of hatred, hates soldiers, not a hero, never uses weapon, never be cruel, never be cowardly, always cares for people, always helping out with only a box and a screwdriver.
Here's where I found this, in a comment on this Youtube video: