He betrayed the kingdom because the situation made it impossible to win the fight. He didn't want to sacrifice the lives of other soldiers anymore and finally he surrendered to the enemy. he came to the castle alone, riding a horse and killing anyone who got in his way to meet you. And accidentally you saw him kill several soldiers right in front of your eyes.

"Sorry princess, I had to betray and side with the enemy. And you have to come with me or your fate will be the same as these poor soldiers."

he said in a cold tone and holding a blood-stained sword