Again, I would like to reiterate here;

Yes, you can argue that they were trespassing and that she was scaling a barrier. In the legal sense, he was right to open fire.

In the reality we all inhabit however, what he did was unnecessary, as the crowd was fairly calm, she was taking her fucking time, and simply dragging her over the barrier and smacking cuffs on her while 2 armed agents covered the opening was self-evidently an INFINITELY WISER course of action. Opening fire however only narrowly failed (as she survived long enough to be taken away by emergency services) to set off a panic reaction (...Well, outside of Congress of course, who were promptly given PTSD by a overreaction of their own security). The result of this would've been simple; a crowd wide fight-or-flight reaction.

The latter would have started a catastrophic escape stampede, most likely wounding dozens and killing multiple. The former however would've seen the rifle-armed agents on the other side of the breach battered to pulp, their arms pilfered, and see the crowd of clueless middleaged white people overstepping during their free tour of the Capitol turn into an ACTUAL vengeful lynch mob bend bloodshed.

The shooter was white. This is disinformation.