How can I argue that Ralph is not responsible for Simon’s death in Lord of the Flies?
Well, you could certainly take the same approach that Piggy and Ralph took in the novel, which is staunch denial that they were involved.
When Ralph first broaches the subject of Simon with Piggy in Chapter Ten, Piggy practically refuses to discuss it:
"'You stop it!' said Piggy, shrilly. 'What good're you doing talking like that?'" (156)
Instead, Piggy points out that the circumstances were to blame: "There was that--that bloody dance. There was lightning and thunder and rain" (156). Piggy's suggestion does have merit. The frightening weather with the combination of thunder and lightning did intensify the boys' emotions during the dance. The storm made them even more fearful of the beast than they were before, and the sum of those fears turned into violence during the dance.
Piggy's argument moves then from the logic placing the blame on the faulty weather conditions and low visibility to presenting the idea of location: the inner circle and the outer circle of the dance. The boys on the outer circle, according to Piggy, did not participate in the violence against Simon.
The more Ralph listens to Piggy find ways to absolve their guilt for what happened to their friend, the more he starts to agree with Piggy. Ralph and Piggy both agree not to mention any of their involvement in the dance to Samneric, reemphasizing the fact that it was dark and they were on the outside, of course.