not wanting to date someone doesn't make you an asshole. It might make her sad (or mad), but you have no obligations to date someone after you kissed them, especially if they didn't make their intentions clear from the start.
Then why did you kiss her? Was it on impulse? Tell her how you feel. You acted on her question to kiss her and you have to take responsibility for what you did. That means being honest, whether you will hurt her or not.