If a person who is observing a mitzvah will not be harmed, how is it possible that Hevel was murdered as a result of the Korban he offered?
In the sefer Asicha Rav Chaim kanievsky answered because Hevel wasn't commanded in bringing a korban therefore he wasn't protected. It's brought from Maharal (there in side notes) that Hevel wasn't protected because he was influenced by someone else to do the mitzva. (In this case kayin). A mitzva only protects when you enlight yourself to do the mitzva.