He reacts purely out of reflex. No sooner do the words leave her mouth are his feet on the floor, and he's poised to attack with his sword. Startling a dude with a stabby kill object doesn't usually end well for most people, but thankfully Zer0 is a professional and he's perfectly capable of controlling himself. When he feels like it. Self control wasn't as necessary on Pandora, because Pandora, but he's being a little more cautious here.

And it's a good thing that he is, because he was about two seconds from probably murdering Angel again. As soon as he realizes that he recognizes the voice and gets a good look at her face, he slowly straightens up, lowering his weapon while his helmet displays a very helpful '?'."

"Well, this is awkward. / Normally my kills stay killed. / ...Have you changed your hair?"

Not the important takeaway here, he knows, but also dude you have horns.