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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The Jedi Order was bloated and misguided by that point, so I don't exactly feel a lot of pity for them. They were using a slave army of clones for -blam!- sake.