I'm already beginning to see, in response to the massacre in Paris conducted by Islamic extremists, blanket condemnations by some Christians of all Muslims.  First this made me think of  Anders Breivik, the pro-Christian pro-Israel right-wing anti-Islamic extremist who murdered 77 people in Norway in 2011.  Then it made me think of Numbers chapter 25 in the Hebrew scriptures:

"While Israel was staying in Shittim, the men began to indulge in sexual immorality with Moabite women, who invited them to the sacrifices to their gods. The people ate the sacrificial meal and bowed down before these gods. So Israel yoked themselves to the Baal of Peor. And the Lord’s anger burned against them.

The Lord said to Moses, 'Take all the leaders of these people, kill them and expose them in broad daylight before the Lord, so that the Lord’s fierce anger may turn away from Israel.'

So Moses said to Israel’s judges, 'Each of you must put to death those of your people who have yoked themselves to the Baal of Peor.'

Then an Israelite man brought into the camp a Midianite woman right before the eyes of Moses and the whole assembly of Israel while they were weeping at the entrance to the tent of meeting. When Phinehas son of Eleazar, the son of Aaron, the priest, saw this, he left the assembly, took a spear in his hand and followed the Israelite into the tent. He drove the spear into both of them, right through the Israelite man and into the woman’s stomach. Then the plague against the Israelites was stopped; but those who died in the plague numbered 24,000.

The Lord said to Moses, 'Phinehas son of Eleazar, the son of Aaron, the priest, has turned my anger away from the Israelites. Since he was as zealous for my honor among them as I am, I did not put an end to them in my zeal. Therefore tell him I am making my covenant of peace with him. He and his descendants will have a covenant of a lasting priesthood, because he was zealous for the honor of his God and made atonement for the Israelites.'”

Then I recalled that the guy who wrote half of the New Testament had previously been a violent religious extremist, until he experienced a dramatic revelation that completely changed his way of thinking.
And there are the Hindu nationalists in India and the machine gun toting Buddhist monks in Sri Lanka. Etc., etc.

The sad fact is that every religion (including atheism) has its violent extremists and often this violence comes as a result of a toxic mixture of religion with political, cultural, economic and ideological extremism.