Have they? When? The norm for Christians when oppressed is to go underground and do such radical stuff as receive smuggled Bibles, etc. Christians are being oppressed in many countries in the world today, most notably but not exclusively Muslim dominated ones. Martydom for Christians has always been a submissive thing. Where Christians have displayed behaviour similar to the Islamist extremists of today is either in olden times when theocratic rulers would yell for God and country and go to war (for example, the Crusades) or in modern times when extremist Christians in, say, the Bible Belt of the USA try to set fire to abortion clinics and the like.
The IRA weren't terrorists? The Catholics fighting against repression in South America - Argentina, Bolivia, Brazil, Chile, Paraguay, Peru, Uruguay - didn't commit terrorism? The ANC didn't carry out terrorism? Presumably you've never heard of that little continent known as Africa, or groups like the LRA? The genocide in Rwanda didn't occur? The Iron Guard? Hutatree? NLFT? NSCN? Concerned Christians? Oppressed or not we can look at examples of Christian terrorism, it exists.
Not only have Christians committed terrorism, but they do it specifically in the name of Christianity too. In Nigeria the Movement for the Emancipation of the Niger Delta, a Christian group, commits terrorism regularly and are clear in that they're "defending" Christianity(from Muslims).
You mention Muslim dominated countries. Are there not Christians in Hamas and Fatah? Hamas has had Christian leaders. Dr. Wadih Haddad, George Habash, Chris Al-Bandak, and so on are examples of Christians linked to or guilty of terrorism. Habash, who created the Popular Front for the Liberation of Palestine, all but invented the now famed terrorist tactic of hijacking planes. It's a lesser known fact that terrorist groups in these regions have Christian members. There are Christians who go a bit further than smuggling bibles.
Nobody is saying there's exact symmetry, Christianity generally is more liberal and reasonable than Islam, western atheism has made sure of that. But that doesn't exclude Christians from acting horrendously, especially when they're being slaughtered and/or oppressed by foreign or domestic enemies. Therefore you'd have to conclude that if Christianity wasn't the dominant powerforce in the world, if the shoe was on the other foot, that Islamic terrorism would drop whereas Christian terrorism would rise. I don't think that's an unreasonable point of view.
And I'm not suggesting for one moment there would be no Islamic terrorism if we changed our foreign policy, if that's what you think.