Reasons (5)

Billy wondered if he had been childish. Svan, despite being brainwashed, came to attack him alone because he wanted a fair fight, but Billy ignored that. While magic was a part of his fighting style, the way he used it seemed a bit cheap… At least that was how he felt after defeating the big guy, and with no sense of accomplishment whatsoever.

The other barbarians didn't even see what had happened due to the darkness of the night, and when the stone bullets fell, the sound wasn't heard because of the thud Svan caused by falling. So, from their perspective, Billy had defeated their leader without using a single weapon, and in a manner so fast that was beyond the speed their eyes could follow. Those moments of shock gave Gilles even more time to knock them out.