The reasoning is classic in its clarity. If God does not exist, Kirilov is god. If God does not exist, Kirilov must kill himself. Kirilov must therefore kill himself to become god. That logic is...

He was one of the numerous and varied legion of dullards, of half-animated abortions, conceited, half-educated coxcombs, who attach themselves to the idea most in fashion only to vulgarize it and who caricature every cause...

I am a fool with a heart but no brains, and you are a fool with brains but no heart; and we’re both unhappy, and we both suffer.