Yes the act was a little extreme, but in the end it was justified. The Jedi attempted an illegal coup against the legal head of state of the Republic just because of his religion. To make that even worse Windu was going to kill the guy without a trial just because "he's to dangerous" and mainly because of his religion.
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Except for when the Head of State illegally gives himself longer terms, creates an illegal army, controlled both factions in the war, Committed genocide against a religion, and made himself dictator.