The Guard knew that, in essence, he had two options only. He could either intentionally lose against the King and die a painful death at Azekiel's hands, or he could fight back and defeat the King, which wasn't hard as all the guards knew that the King was quite bad when it came to actually fighting.
It was just a rumor in the Kingdom that he himself spread about him being a great warrior who could defeat many enemies at once, but in reality, he was nowhere close to that.
The Guard was clear that if the King had to win, then he could only do so if he intentionally lost. But in that case, he would get no benefits, and his family would also live a struggling life. And if he killed the King, according to Azekiel, he was going to become the next King! There was no question left in his head as to what he had to do.