ANGER
Yuan changes a lot after Martel is killed during the Kharlan War. He almost loses his mind because of the irony of her death. Martel wanted to fight in order to save the humans, but in the end, they were the ones who killed her because of what she was, a half-elf. This is the pity of the game, the fact that humans and elves are so racist against each other, and especially against the half-elves, that no one truly belongs anywhere. The vision Martel had was to combine the two sides and find a place for everyone to fit in, including them. I think that the death of Martel caused Yuan to lose complete hope in the world and vision Martel had, and were ultimately fighting to protect. Sadly, even though Martel wanted them to keep protecting it after her death, the anger Yuan felt totally eliminated all of Martel's wishes in his mind. Yuan, not being the only one to feel this anger, joined forces with Mithos and Kratos, following a new set of idea to separate the worlds and bring Martel back to life.
Anger and hatred towards a group of people for hurting you or a loved one is never a good thing. Yuan was too hurt to understand that he needed to take a deep breath and consider what he was about to get himself into. He was about to change history by following Mithos, a person who was also devastated, for the worse. However, at the time it honestly seemed like the only logical option. Obviously this is a far stretch, but when people are in a loss of what to do, they follow a leader who could be a monster because of what they are promising. Mithos is almost like Hitler. The Germans were devastated after the first World War, broke, humiliated, hated, and Hitler came and promised a change for the Germans. However, the change he brought ended up destroying and killing many innocent people, just like Mithos is about to do... Yuan is not really a leader at this point because he is too angry, so he becomes a follower of a horrible cause.