If federal security at Court House shoots BLM protestors who are entering a federal court house, those security people would probably get charged with murder.
It's unreasonable to expect Capitol Police to make that sort of moral choice in the moment. And if you give cops the greenlight to shoot people to protect property, there will be a lot of unnecessarily death going forward.
That said, they may (probably?) screwed up containing the crowd contained in the first place. Though to play devils advocate, the President had just told a mob to go "wild." Not sure if Capitol Police could successfully manage that.