character development - How does my secondary protagonist turn into the antagonist?

My two main characters are twin souls. One soul is Yang, the other is Yin (Positive/Negative personalities). Both are 15 year-old orphan teenagers sharing the bond of love.

The problem is, I want to cut that bond and turn it into hatred. The Yang soul is the girl, she is grateful to my Yin anti-hero for saving her life.

They run away together from the orphanage and join a martial arts dojo where their sensei takes care of them. He trains both to become exorcists. 5 years later, the sensei dies at the hands of a demigoddess succubus and my characters are the only survivors after a bloody battle. The dying demoness reveals that my Yang protagonist is a demon-human hybrid and that she is one of his many demon half sisters, then she dies. Then both of my protagonists start their journey.

How can I turn my female second protagonist into the antagonist during the journey?

(Note: My Yang protagonist is very possessive of his twin soul. He is misanthropic, quiet and peaceful when not disturbed, but he gets angry easily and enjoys torturing his enemies killing them regardless if the enemy is demon or human. He values justice above all things because bandits killed his family for a relic that they sold to a lesser demon. The female Yin protagonist is the opposite of him. The only thing they have in common is mutual hatred for demons. However she may befriend a demonic prince in the future.)

Answer

The difference in attitude between the two main characters already has plenty of potential for conflict. Yin's tendency to commit acts of pointless cruelty (even if against enemies) will certainly not be approved by Yang and will stress their relationship, regardless of gratefulness for earlier deeds and family bonds. At one point Yin might do something so horrible that Yang will feel that they have grown apart and distance herself from him.

Now how do you turn Yang into the antagonist after that split?

Considering that the communication between Yin and Yang will now be limited, they will have different information regarding the demon threat. This will cause them to act in ways which seem to contradict the goals of the other. It might cause either one to believe that the other is working with the enemy. This hypothesis will be plausible for both sides. Yin might be working with the demons because of his violent outbursts which make him look more dangerous to Yang than an actual demon at times. Yang might be because of what Yin perceives as weakness and lenience towards the enemy. The last straw might be when Yin hears that Yang befriended a demon prince. Then he will be certain that she turned traitor.

This is a typical tragedy setup. All characters have good intentions, but their personality conflicts stand in the way of cooperation, cause them to work against each other and end in harm despite everyone just wanting the best for everyone.

Note that if you want the reader to identify with Yin and perceive Yang as the enemy (at least for a part of the story), you will have to make Yin the primary viewpoint character and not yet provide the reader with all the information which explains Yang's actions.