On the new chapter-
Spoiler:
Yikes, that puts Dragon Slayer magic at the top of the power charts again! Well, barring Zeref. The Zeref and Dragon plots seem to be getting more intertwined.
Originally posted by socool8520
He looks determined herehttp://wootmanga.com/fairy-tail/296/5/
He also looks noticeably shocked when Natsu and Gazille get back up from the dragon force barrage.
Besides, Natsu assumes he can beat everyone as well, but that doesn't stop him from going all out when the battle is on the line. There is really nothing to suggest that Sting wasn't giving it his all, and even remarks that Natsu was just too strong.
He was giving it his all with those emotions, but take a look at the next page- he's basically trying to justify why this level of power should be working. "This killed a dragon, so it should kill you!". He's not digging deeper, he's making excuses and relying on his power over his emotions.