Im not sure how to explain this one...
The man was deeply devoted to his oath as well as protecting her. He gave up all his ambitions in life for his oath and service to her family. Everything. Any chance of a family, riches, love life, a normal life in general. When she came on to him he thought it was more than just a fling. He thought she genuinely cared for him. When he found out he happened to be the closest dick around that shit hurt to his core. He had betrayed his entire life just to be a noble's plaything.
Its a classic theme, but with the genders reversed. Shocker, men can kill emotionally abused and manipulated as well when objectified by people who hold authority over them. Its that simple, he felt emotionally abused. He killed the other guy out of hurt, anger, frustration towards the games of thrones and how the guy tried to just normalize the way nobles see them. The man was on edge, obviously suicidal, and the guy he killed was the last straw.
Top of my head, my guy was Monica Lewinsky.