Neville came up behind the basilisk only moments before it struck. He only had time to see it rear it's head back before he was aiming his wand. But he hadn't been fast enough, it jolted forward at impossible speed and had hold on Elektra's ankle by the time the green jet of light rushed from his wand and lit the whole space around them.
He should have done it earlier. In the blip of a moment between realizing what it was and getting to safety, in the time between Elektra leaving him and the snake passing him. But there was no way to know if it even would have worked. Casting the killing curse was difficult enough, having to do it non-verbally made that worse, between the surprise and disorientation, having to keep his eyes shut... He had been planning to do something else, try to slow it down or blind it. But seeing the snake attack Elektra had been enough. It dropped just as quickly as it had struck out and Neville was rushing past. He crashed to the ground at Elektra's side, hoping it hadn't made contact or hadn't hit hard enough to puncture skin.
The blood rushing from her ankle quickly doused that hope. He would have cursed, could he speak. He, very carefully, put a hand on her wrist first letting her know that it was him, not something more dangerous. She didn't have long. He quickly pulled off his shirt and ripped part of it off, wrapping it around her ankle.