Question

why did aurangzeb kill his father and his brothers for no reason. why was he so cruel

Answer 1

Their is always a reason behind a crime.
He killed his father and his brother to make his way to the throne. He wanted to be the heir of the Mughal empire by killing his his own blood.
Greed for wealth, fame and glory can make any person such cruel as Auranzeb.

Answer 2

Aurangzeb prison his father....and killed his brothers for his greed to become the emperor of Mughal empire . he was just lost in his greed that whoever came in his  way to the throne he killed everyone...no matter it was his brother or father...he killed his brothers in the battle