Houses with flat roofs were common in ancient times and there was a danger that accidents could occur and someone might fall off. The Torah therefore requires that a roof have a parapet as a safeguard (Deut. 22:8).
But no safeguard can guarantee complete safety. As the Talmud puts it, “A drunken person falls by himself” (Shabbat 32a).
Parapet or not, the person who does not know what he is doing is still going to fall. So why must I go the expense of installing a parapet?
To ensure that I am not guilty of even the slightest negligence. Even if it is more the fault of the person who falls, I must feel clear in my conscience that I did not contribute towards his fate.
This is also the message of the High Holyday confessions. Ashamnu, bagadnu, gazalnu”– “We have sinned”, “we”, not “I”. Al Chet – “For the sin we have committed before You”, “we”, not “I”.
Whatever the sin, it is the fault both of the sinner and of the rest of us who might have saved him or her from succumbing had we been careful enough and made certain there were enough metaphorical parapets to make it more difficult for anyone to fall.