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.
-
You gotta realize the emperor started the clone wars though. It was always his plan so for him to win it was justified and an inevitability. Mace windu was a thug though, he lives by the law of the streets.