The Twitter video that I saw shows the protesters already in. The guards are allowing them to stay in and asking them to remain peaceful, but it doesn’t show them opening the doors to let them in.
I think the killing of Ashli Barrett was outrageous because she was shot from ambush after the police on her side of the windows had stood by while the windows were broken, and then stepped back, giving her the reasonable impression she could continue without being harmed. Justice would be served by sentencing her killer Byrd to prison for a long time.
I have no problem with the use of deadly force to control rioters, though, if done with proper warning and documented well with videos. I believe the Capitol could have been protected without killing anyone — or, most likely, even harming anyone — if the police had formed a line in front, drawn their guns, and announced that anyone who approached them would be shot. (I see no reason why police should have to wrestle with demonstrators, as is routinely done around the world.)
How many persons would have been suicidal enough to advance into drawn weapons after such a warning? Probably none (and if they did, I’d have no sympathy for them). They had the right to protest “peacefully”, yes — as Trump asked them to — but not the right to occupy the Capitol.