And then when the country's all Muslim, they start killing each other. Christians has been through this but at a time when WMDs are not available yet and Christian sects have all eventually reformed.
There are a few 100% Muslim countries around the world, such as Afghanistan, Saudi Arabia, Yemen, Sudan.etc. All these countries are currently affected by civil war, perhaps with the exception of Saudi Arabia.
Saudi don't yet have a civil war because they are the ones spreading radicalism. They donate a mosque to your area and you know what's coming next. They've had a few attacks though but those are mostly targeted against the Shia minority there.