And for all of the transgressions of the Torah is not the whole world punished? Isn’t it written: “And they shall stumble one upon another”?! (Leviticus 26:37) [That is to say,] one because of the iniquity of the other. This teaches us that all Israel are sureties (responsible) for another! There [they are punished], because it was in their power to prevent [the sin], and they did not prevent it.
Shevuot 39a describes the need for all Israel is protect and defend one another.