Harming or killing anyone isn't important, what's important is what the person claiming defence of others believed and was that reasonable.
“It’s twenty minutes to six. I’m not in a particular rush, and I actually enjoy the idea of some field work. We can talk for five minutes, and then have ‘Alexandria’ remove one of your teammates from the field. Depending on the situation, I will either arrest them and take them to PRT offices in New York and Boston, or I’ll kill them.”
[...]
My head turned as I sensed the truck arriving. Bugs clustered to it as it found a spot at the side of the building, PRT uniforms moving their target on a stretcher. My bugs shifted position, tracking what they were moving. The white mask, the curls, the shirt, with a tightly woven fabric beneath… a spider silk shirt? It was Regent, unconscious.
The bugs moved, tracing down the length of his arm. It was broken in two places, virtually zig-zagging. His leg was the same. I caught the words ‘medical’ and ‘doctor’. ‘Tranquilizer’.
[...]
The PRT van arrived before Alexandria did, this time. The PRT officers took their time before they finally emerged.
Restless, I shifted position. My forearms were digging into the table where the cuffs held my arms in front of me. Leaning forward made my back hurt.
They opened the back of the van, and there were no precautions this time. Their weapons were holstered, and they didn’t stand in any particular formation.
The only thing in the back was a single body bag.
Back pain forgotten, I went as stiff as Tagg. I drew more bugs in to get a better picture of the scene.
They took a different route this time, using their phones to make their way through the various doors and checkpoints. They entered the attached building. A small hospital, or a special office with medical facilities. It didn’t matter.
And they made their way to the morgue.
The size of the bag… it couldn’t be Lizardtail. He was too large. Was the wrong size and shape to be one of the dogs. That left only three real possibilities. A PRT officer, Rachel, or Brian.
I closed my eyes, clenched my hands.
Then one of the PRT officers who was escorting the body said something, and others laughed in response. It was one of those rare moments I could discern tone, and it wasn’t a kind one. That was enough for me to know it wasn’t one of them. It wasn’t a civilian.
So form Taylor's perspective Alexandria uses her secret identity to threaten to kill the undersiders, then goes out and breaks regents arm, threatens some more, got out and comes back with Imp deliberately not letting Taylor see her, then issues an ultimatum, when Taylor rejects it she leaves again and the PRT show up with a body bag making jokes.
Even if you could claim it's not defence of others, Alexandria threatened to kill them and one of them died temporary insanity is a plea that works when used, especially when someone just lost someone they loved and lashes out at the person/group responsible. It's not a get out of jail free, but it's a strong mitigating factor against recivism that often sees cases resolved as time served or otherwise minor requirements/punishments(attending anger management classes, seeing a therapist, grief councilling, etc).
Taylor has a huge rap sheet so she's clearly still going to juvi, but if it was just Alexandria/Tagg it'd be hard to argue she wasn't acting in the defence of others and out of grief(although her little "how was it she put it, inevitable" speech probably makes that harder to argue that can be explained as her power suppressing displays of emotions).