Acharya Prashant clarifies the fundamental difference between criticism and slander, particularly in the context of spiritual teachers. He defines slander as the act of falsely accusing another to protect one's own ego. Slander originates from self-preservation, where an individual labels anything different from themselves as 'wrong' or 'bad' to maintain the illusion that they are already correct. He explains that slandering a saint is considered a great sin because it traps the individual in their current state of suffering. Since the slanderer insists they are right, they lose the opportunity to change, which is the ultimate punishment.