Why do people insist that their belief is true and beliefs followed by others are false ?

Beliefs and religion are good indicators of morality. They are often so complex they have conflicting parts. But that is what makes it such a useful tool. Evil people will insist on emphasizing the evil parts, (kill gays and disobedient children, etc...) a good person will emphasize the good parts (love each other, forgiveness, etc...)
So when someone insists on their own beliefs being true at the exclusion of all others its a red flag they are actually the bad guys.

Of course the good guys can insist they are right as well. But in that case they are right. It depends on what they have chosen to emphasize.