Dear Mr. Sidqi,
Quran raises all the noble companions of prophet Mohammed in high honor, so those who were brought up by him or grew up in Deen with him or learned Deen under his auspices are no exception. We should only reject history that defies common sense or is unappealing to the mind of a reasonable person. But, with due respect, rejecting just everything based on the fact that there is no solid evidence will bring up this question? Do we all have a proof that we are the sons of your fathers except that our mothers say so. I hope you get the point.
We can thus derive from common sense that since Hazrat Hussain kept a good company, the company of noble companions of exalted prophet, he believed and understood Quran and the divine values Quran encompasses and therefore led a life and government based on Quranic principals. Even if Assassins of Hazrat Hussain were parsis, the assasination was not a personal revenge but a political act to destabilize the then Islamic government something that could only be achieved by eliminating the noble leaders of that era.