Levi had one principle: Rather get 1000 killed than get one missed. He tortured the soldier who had sneakily escaped in the middle of the night.
This soldier was indeed a spy. He was a spy that the Bird of Death's Voice had bribed to facilitate the assassination.
It had been a while since he had snuck into Black Water Valley. When Sir Fred left the castle, he was the one who reported to the Broker of the Bird of Death's Voice.
Then, the assassin took advantage of Sir Fred's departure to sneak into Levi's castle at night and assassinate him.
Of course, this spy did not tell the Broker that Levi had already become a knight. This was a fatal mistake on his part that caused the failure of the assassination.
However, this spy did not know who ordered his assassination.
The Bird of Death's Voice was a professional team, after all. They did an excellent job of keeping their customers' secrets.