We still don't know who fired that shot, and there were no reports of shots fired otherwise. Given the scope of what happened, a single lethal shot fired is kind of a miracle. And shocking given how many injuries and deaths we saw from protests earlier in the year which were under far better control, with far more law enforcement involved.
If you poke around a bit on YouTube, there are multiple videos of it happening on there. My guess is that the woman was trying to get into a corridor that the Secret Service was protecting, and they really didn't want anyone in there. Capitol Police seemed unaware of this as a few of them armed with long guns stood on the same side of the door literally right next to the woman who was shot, when she was shot after trying to enter through that broken window.
This article gives some background color on her public social media. [1] It's pretty depressing honestly to think that she was so caught up in conspiracies and it got her killed.