Is the report that Sa'id ibn al-'As killed the people of Tabaristan after granting them safe passage authentic, and if so, is this act considered a betrayal of trust?

The summary of the fatwa is that the report regarding Sa'id ibn al-'As's invasion of Tabaristan and the claim that he killed all the people of the fortress except one man is very weak and unreliable. This is because the chain of narration for this account includes Ali ibn Mujahid al-Kabuli, who is accused of lying and fabricating hadith, and also because no narrator named Hanash ibn Malik, to whom the report is attributed, has been found.