It was at the locked corridor barrier to the Speakers Lobby which adjoins the House Chamber. Rumors that Pence was there and it was Secret Service haven't been confirmed; he had been in the Senate chamber and Secret Service had evacuated him, so it's unlikely he would end up on the House side in a poorly secured area. News reports indicate it was simply Capitol Police, possibly someone associated with the House Sgt at Arms since they have an office somewhere around there.

I guess the woman, despite being a veteran (Air Force... why is it the Air Force has so many crazy people?), was so caught up in LARPing that she couldn't rationally evaluate that this was the last meaningful barrier to the House chamber and they might have more liberal rules of engagement or someone might panic and overstep their RoE to prevent an unauthorized mob from entering. If she'd climbed through, she could have opened the door to the mob.

On the other hand, I can see the argument that shooting her was an absurd overreaction. There were armed capitol police who had just come up the stairs behind her and that group of protesters, and they were doing nothing. The news reports that the justification for the shooting was that she might have been armed, but if that's really their rationale, it's a bad one. They should have retreated to the main chamber, because they weren't in a good tactical position, trying to defend a barrier with windows large enough for people to climb through. In addition, no such resistance was offered on the Senate side. When you're outnumbered that badly, unless you have special training like the Secret Service would, and plenty of ammo, you're not going to accomplish anything by shooting anyone, unless they are overtly threatening people such that you think you're going to die or be taken captive anyway.

I don't think Capitol Police are legally at fault in any way for her death, but I think they did the wrong thing there.

I'm hesitant to speculate too much before there's been an investigation, but I believe the shooting took place when there were still a number of congresspeople in the gallery of the house chamber, and she appeared to have a large backpack on that obviously didn't go through security screening.

I can imagine the Capitol Police being worried that something very bad could happen if she was allowed to breach the barrier and run into the chamber.

According to that huffpost reporter, they were still in the chambers at that moment, and the shooting was less than 20 feet away.

The video footage from inside the chambers at the time of the shooting showed people were still in chambers, mostly up in the second floor gallery, but a few on the floor.

I don't have a good phrase for people that willfully chose to believe something deranged. But believing that plainclothes security is going to let a mob get their hands on a congressman, that's it.

I agree with you, these folks charging through the Capital and expecting to get away with it were completely deranged.

She was carrying a backpack and attempting to breach a security perimeter.

It’s a good shoot pretty much no matter what else at that point. The police officer can reasonably defend that perimeter with deadly force.

Regrettable that it came to that, no one wants to see anyone due, but if you don’t want to get shot don’t go to a riot.