Breivik isn't a Christian terrorist. He described himself as not being overly religious. He is also very critical of modern (I think that's what's meant by Western here) Christianity, both in the Church of Norway and Catholicism.

I've read somewhere that the Charlie Hebdo attackers weren't actually very religious either. Didn't visit mosques or read the Quran, but felt separated from society, and looking for some meaning to their life, some way to have impact, they found this. The article claimed they had more in common with Breivik and the Unabomber, or maybe even the Sandy Hook shooter and similar lone gunmen.

It wouldn't surprise me if the same is true for many extremists. ISIS also apparently gets a surprising number of recruits from Europe. People not fitting in looking for a place where they can be appreciated.

Muslim terrorists in general are also in opposition to mainstream Islam. They are extremists, just as Breivik is an extremist.

Wahhabism and other extremist versions of Islam have a significant following. Polls regularly indicate support or acceptance towards extremist views within Muslim communities. It's therefore not as simple.

I don't think one Christian terrorist is enough to say that Western Christianity "certainly" is violent. I wouldn't even say that about Islam even though I think Islam has some problems with violence that Western Christianity doesn't have.

I'm not saying western Christianity in general is violent! Certainly Breivik is a lone extremist with absolutely no support by any mainstream church. But it doesn't change the fact that he was a christian and what he considered christian values (in his own eccentric interpretation) is part of what he believed he fought for. That makes him a christian terrorist, unless you apply some kind of "no true scotscman" filter, which could just as well be used to argue that no muslim terrorists are actually muslim terrorists.

Another example would be the IRA.

You're right and I agree that he was a Christian terrorist in that sense. I was mostly commenting the use of "certainly".