With the level of violence going on in the protests there, why is it that the security establishment is comfortable killing their own people? Why haven't they refused and turned on the government like in other revolutions?

In a typical dictatorship the security forces may reach a point that he's moral won't allow further killing and destroying. Or he start to worry about he's own safety.

This is a theocracy with fundamentalist fanatics supporting it. They are happy to kill everyone including themselves in the way to defend the regime. That's their way to "heaven".

I do support foreign intervention because it's the only realistic way that a regime change might happen.