Without a transcendent guarantor of morality, there is no binding ground for ethical obligation, so every act becomes permissible in principle. The argument is advanced as a logical consequence of unbelief, not endorsed as a way to live, and it is shown corrupting the man who absorbs it secondhand.
Smerdyakov, the illegitimate servant son, physically kills old Fyodor Karamazov after concluding from Ivan's intellectual arguments that nothing is forbidden, then names Ivan as the true author of the crime. The hot-tempered son Dmitri, who openly wished his father dead and was present nearby, is convicted instead, so the man with the motive and the man with the idea both escape the legal verdict that falls on the innocent.
Even if suffering is ultimately redeemed in some final harmony, one can refuse that harmony on moral grounds if its price is the unavenged torture of a single innocent child. The protest accepts God's existence yet rejects His world, locating rebellion not in denying meaning but in declining to accept meaning bought at that cost.
Responsibility is treated as shared and total rather than partitioned: every person is answerable for everyone's sins, because no one's cruelty or neglect is sealed off from the lives around it. Seen this way, active love and self-accusation replace blame-assignment as the response to others' wrongdoing.