Are Islamic terrorists an "existential threat" to Western Civilization?

Yes especially ones funded by our government.

Existential threat to Western civilization - no, that overstates it. Existential threat within their own neighborhood (the Middle East) though - definitely. Particularly given the lack of direction in most Arab countries (remember how all Western politicians of all stripes were celebrating the death of Gadhafi - check out Libya now).

Islamic terrorists are very dangerous and capable of the occasional very vicious attack in the West? Yes, unfortunately. Every so often the rats will emerge from the gutter and get their headlines, we will all panic like crazy for a couple of weeks and then peace will be restored .... for the time being.

Not really. They are a threat to the security of individuals within Western civilisation. But not to western civilisation as a whole

But the thing is that Islamic terrorism thrives in precisely the places where western powers have interfered in the Islamic world. They see the west as an existential threat to them.

The best way to avoid future terrorism is to stop playing world police

That depends on what you mean by Existential. Are you referring to Green Lantern or Ant Man?

Existential in this case means a threat to our existence.

Hitler was such a threat. Stalin was such a threat.

These guys are not such a threat.

I've never been threatened by an Islamic terrorist. I have been threatened by Bush's surveillance state however.

No. They're just pissed of religious lunatics.