Where did I lump all Muslims into one group? All Muslims are not terrorists, but most terrorists are Muslim.
You're probably correct with that statement, as long as you add "currently".
In the 70's, most terrorists were left wing extremists.
During the 1980's in Latin America, there were quite a few right wing terrorists.
Right now though, the whole terrorist trademark is pretty much owned by radical Islamists.
BTW, the whole concept of why (some) extreme Islamists decides on using terrorism and how they defend that is a topic in itself.
How can people who claim to be of Islamic Faith defend that they're killing other people?