It's not that it's strange for her to want to kill Light, it's that the scene itself is just done so bizarrely. She picks him up to carry him away, then suddenly she's about to kill him, with no transition whatsoever in the mood or blocking. It's the middle of a losing fight with powerful foes - her primary objective at first appears to be getting the fuck out of Dodge, which is just about the most *sensible* possible thing she could be doing, but then suddenly she's burning precious time and energy dragging Light away to kill him, but not quite far enough to not be noticed and followed by not one, but *two* Leaguers. The dialog doesn't flow or make any kind of sense.
That she'd have no problem snuffing him suits any Cheetah. That she'd actually exert some effort and energy to do so suits several Cheetahs. That she'd suddenly try to do so at the worst possible moment, the most inconvenient and dangerous time and place she could pick, after willingly coexisting with him for however long, suits, like, Joker. Maybe. Even he's not usually that random.