The set-up of the steal is wonderful - with the guard having already stripped of her casual clothes, and starting to put on her uniform, unaware the intruder is right behind her and about to strike. Love the detail the girl still has her beanie on. Personally, I much prefer the "Modern Age" art styles of comic books compared to the classic "Golden Age" ones. Though it has the side-effect of making some of these scenes a bit frustrating, when the victim is a good-looking girl, but who only has one panel / one page worth of screen time before the plot forgets she even existed. (I'm still not over that cute blonde from the Doctor Aphra issue, who didn't even get an aftermath scene...

Or that goth girl dressed as an Alice henchwoman in that issue of Spider-Man.) Authors come up with cute/good designs that we barely see... And it's a bit true with this guard as well.
It's a shame most of the scene is left ambiguous (and offscreen). (The full issue can be found on some websites, but doesn't give more clues.) Though, given the general mood of the story, I take it the ambiguity and violence are deliberate.
I guess it's up to interpretation whether it's fatal or not. Still, I believe it's non-lethal. Sure, the character pulls no punches, and the knock-out blow is a bit violent. However, she's also a mole working for the X-Men, who are clear-cut "good guys". Plus, she uses a blunt object to hit the guard, and in fiction it's usually a code to indicate a non-lethal knock-out. (To the point there's even a trope - Tap on the Head - that discusses this narrative convention.) Same with the sound effect. WHUDD doesn't scream neck broken to me - and I've already read issues in which a 'KRAK!' was used to indicate a non-lethal KO (since the girl is bound and gagged afterwards...), so...
All in all, this scene is a great source of inspiration. I must find a way to feature this guard somehow. She'll find out things are a bit gentler in Earth-USB... Though she'll still lose her uniform and dignity, that's non negotiable!

Thanks for sharing, tirepanted.