When Elektra bolted down the hall, Neville was almost angry. He'd understood her words, and trusted her, so he'd started to count in his mind. But then she ran. Was she leaving him there to die? Knowing he couldn't hear it coming and couldn't keep his eyes open to look for it. Was he so expendable that she'd use his death to bide her own time?
He didn't want to think so. They weren't close, but she didn't seem that cold-hearted.
The basilisk was almost on him by the time he noticed it. He shut his eyes quickly and then started counting again. His only hope at that point was not seeing or being seen. Standing there, with his eyes shut and no way of knowing if he was about to die, his mind kept racing. If this worked, if he went unnoticed, then maybe he could get the upper hand. If she could distract the snake enough that it didn't notice him coming up behind it, he could attack without being seen. Hopefully her running would be enough to keep it occupied enough not to noticed his footfalls too, since he'd have no idea how much noise he was actually making.
He hoped that maybe it had been her plan too. But even if it hadn't been her intention, he could at least try to stop it before it could hurt anyone.
He finished counting and, cautiously, opened his eyes. Just in time to see the very tail of the snake slither out of view. His vision was a little better now, having been in the true dark for a minute, but it still wasn't ideal. He didn't have much choice though. He started after the snake, following behind it and hoping his efforts to stay quiet were working.