I personally see it as revenge. Sharptooth just started out hunting for food, but when the thorn in the eye incident occured, he wanted revenge. Evidence, Chomper is proof that sharpteeth have more personality then leaf eaters think, and as Redclaw is actively seeking revenge against the gang, Sharptooth probably is too. If someone stuck me in the eye with a thorn, I would want revenge, so there's no reason it couldn't be revenge.
I completely agree with you.