It's not exactly luck that Natasha is on the scene. For all that she'd intended to stay in and out of the way, when the bloodshed started, Natasha hadn't been able to look the other way. What's happening here? That's just what she's had her eyes and her ears open for--the only real surprise when she responds to a scream and the smell of blood is that she knows both the victim and her attacker.

That recognition doesn't hold her back though. The scent of blood jangled her senses, but Kenzi makes a good surrogate for the aggression that it stirs up, and Natasha pulls her off Nancy and whirls her up against the wall with all her strength. At this point, she's not worried about being gentle.