Whenever you find a guy who compulsively does not allow others to disagree, know that guy is not allowed to disagree himself. There is no exception; but sometimes one can agree so thoroughly that he becomes one with what he originally disagreed with.
Disagreement doesn’t mean hostility. A mere ‘no’, or an expression of a different perspective can be enough (it must be genuine). In fact, if done as lighthearted and simply as that, and if done at the correct source one disagrees with, and not towards irrelevant others, it can nullify the felt need for hostility as well as other consequent problems.
Is there a person, idea, group or other thing you cannot freely disagree with? Is it your boss, money, teacher, reality, good manners, science, sanity, normality, customers, religion, political ideology? Does anything have an objectively always-right, always-true status? Would you be wrong, bad etc if you disagreed? Better handle that before you start generalizing and attacking irrelevant others.