>>17295180Him killing the kid was mere coincidence, and all it did was give him a bad dream. If it wasn't for the assassination mission, he wouldn't have gone to Griffith afterwards and overhear the conversation between him and the princess. Therefore, he wouldn't have ever thought about leaving the Hawks.
Guts is a killer, and that's all he enjoys doing, because the battlefield gives him an edge to survive when circumstances are limited. He didn't see the assassination mission as a dirty deed.