When I want to say someone passed away because of a certain reason (cancer, accidents etc), which one would sound more natural between "pass away from" and "pass away of"?
In the case of cancer it is not so clear cut
He died of cancer.
He died from cancer.
are both used.
However in an accident, it is usual to hear
He died from injuries sustained in an accident.
He died in an accident.
He died of an accident
Also, you would say
He died from (too much) drink. (correct)
He died from drinking. (not correct, but understandable)
He died of drink. (not correct, but understandable)
One can use "of" or "from" in the phrase you are using, depending on the circumstance involved, but more often one hears people employing a phrase as follows:
Someone passed away due to cancer, an accident or some other reason.
6: required or expected in the prescribed, normal, or logical course of events