Benjamin had killed the king.
Earlier, he had taken advantage of the splashing water and shattered ice and used magic to create a hole in the wall. Then, he hid inside the cupboard, used a mirror to create an illusion of himself, controlled the illusion to look as if it was escaping, and fooled everyone.
Aldrich had scrambled out to chase after Benjamin and left the king alone. And because the king was so enraged from getting stabbed in the thigh, he had ordered all the guards to quickly seal the city and look for Benjamin, leaving him alone in the room with the servant.
So, Benjamin was given the perfect opportunity to emerge from the cupboard and end the life of Ferelden’s current ruler.
In all fairness, he did not kill the king impulsively; what he did had a reason to it. He was very clear that if the king was not dead, the church would be able to implement their plan.
