Dragonslayers are powered by emotions. After just heartlessly killing Sting's best friend in front of him? Yeah, I'm not sure anyone could take that blow.

Also, had he been worth the title of GM he would newer have let it connect in the first place.

I think Guilds have some leeway and I can't help; but think the smirking chick from ST will say something like "In ST a member can challenge the Master to a duel to death for the leadership, and you can also choose someone to fight in your stead. I challenged Master and asked Sting to fight for me"

Or perhaps I am sleep deprived and only make sense in my head.
Whatever internal rules ST might have doesnt matter when they break the rules of the kingdom, and i am pretty sure they have rules about killing your own guild master.