He openly admitted to Wedge Antilles that he used to "Bullseye (kill) womp rats" in his T-16. So when he killed animals for target practice, was this leveraging him to the Dark Side? It really adds to the questionable things he did.
Opinion based answer since I really doubt there's anything in canon:
Even if Luke was using the force to hit the womp rats and not just motor-skills (consider that shooting a rifle is a hell of a lot easier than shooting torpedoes from a fighter, and that shooting on a planet is easier than shooting in space), the Dark Side of the Force requires an emotional component; giving in to a negative emotion and letting it control your actions.
"Bullseyeing" womp rats may have been fun and it may have given Luke a measure of pleasure but he didn't hate the womp rats, and he didn't fear them and think they needed to be killed. The only emotional component to that would have been satisfaction at a 'job well done.' Hence no Dark Side points for Luke.