Is the reward of martyrdom and the vast mercy of Allah sufficient to expiate the sin of turning away at the moment of death, such that Allah may forgive Zayd and Abdullah and admit them to Paradise with their heads held high?

This tradition was narrated by Abd al-Razzaq in his Musannaf and Abu Nu'aym in Hilyat al-Awliya' from the narration of Sa'id ibn al-Musayyab from the Prophet, peace and blessings be upon him. It is a weak mursal hadith due to the weakness of Ali ibn Jud'an in its chain of transmission. Even if it were authentic, there is no problem with its meaning concerning the virtue of martyrdom and the vastness of Allah's mercy, and that Allah may, out of wisdom, deny His servants some of His bounty, such as to show the virtue of Ja'far or to illustrate the seriousness of turning away from advancing in battle. This should inspire fear but not lead to despair from Allah's mercy.