Persuading yourself that a single decaptiation strike on the corporate heads is worth the collateral damage just isn't the same as wanting to personally kill a victim with your own hands. In the first case you can easily ignore the reality of other human beings dying. It's like having no problem with drone strikes. In the second case it a matter of reveling in it. It's like taking fun photos at Abu Grhaib. The mentalities are emotional opposites.
Also, the elaborately militarized tactics Liber8 uses require extensive training and combat experience. In the nature of things it is not yet obvious how the members could possibly have acquired this in the corporate future.
Willing suspension of disbelief is a tricky beast. It wants to be fed, because it's hungry for story. So at this moment (second episode in) I'm happy enough that Kiera gets caught out as not really being Linda Williams. Brian Markinson not taking her word for it and never talking to Portland is just too in your face to easily accept. (Unfortunately the issue of the US-Canada border doesn't go away till the corporations take over. But still....)
But the big problem for the Liber8 members is not their attitudes to and capacities for violence. The big problem is they planned on getting caught, relying on someone they had no reason to trust, to help them escape.