What is the justification for pointing guns at people who are just minding their own business?

It’s a performance piece. ICE acts like this because they are pretending that they are actually the ones under threat. That they are really fighting to survive in a lawless land where every man’s hand is against them. The truth, of course, is that the main threat to anyone in the area is ICE itself, but they have to maintain the optics otherwise the media might wake up and realize that ICE’s main opposition in Portland are people dancing around in inflatable frog costumes.