User blog comment:Water Dragon Slayer/DREAM BATTLE CHRONICLES: SEVENTH MATCH!/@comment-78.146.106.40-20130829204638/@comment-7418318-20130830082736

I agree with the anon, we don't know how F-Rogue killed Sting.

I doubt the two friends would have fought in a fair battle. More likely Rogues attack caught Sting by surprise or he was dead before he knew what was happening.

Or it might even have been something like with their Dragon parents with Sting being crippled in the initial Dragon invasion and asking to be killed by Rogue.