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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I see. So it would be justified killing American colonists since they moved over religion and rebelled? COol to kill the Jews and minorities because of religion and that many illegally fought the government during WWII. All those slaves fleeing from Egypt? How dare they illegally disregard authority! They think they can break the law just because of their religion? Absurd, right?