The real answer to this question requires a further definition of which version of Islam we are talking about.

Shia vs. Sunni would be a good place to start, but even answering that would be incomplete.

Within each there are a range of sub-groups. For example, within Sunni Islam, there are several mainline schools of thought like the Hanafi and Maliki schools. Within Shiism you could find Twelvers, Druze and other groups.

The overwhelming majority of Muslims I know are peaceful, loving, good people who just want to raise their families, work and provide for themselves, and live in peace.

As in most religions, you then find "fundamentalists," people who are convinced that anyone and everyone who does not believe exactly as they do are not only wrong, but a threat that must be stopped somehow.

A subset of the fundamentalists are the violent. Those are the ones to be concerned about, and who generally constitute a minority in the world, whether discussing Islam or any other faith.

That said, there are Islamic groups (some of the Wahabis come to mind) that preach a violent overthrow of those in power, both where they live as well as around the world. They are atypical, but they are also vocal, violent, and intimidating.