Rashi applies here a principle--that the Torah teaches to kill someone who seeks to kill you--from a discussion in the Babylonian Talmud, Sanhedrin 72a. Do you agree with this logic? Are all thieves willing to become murderers?
Sforno on Deuteronomy 22:8
If it were to happen that someone falls from there you would not be the indirect cause [since you had put up a protective railing]. Had you not done so, your family might bear part of the bloodguilt for such a mishap.