It's probably a character flaw, but Will's instinct when he sees people running away from something – faces twisted in terror and legs pistoning like they can't control them – is to go see what they were running from. It turns out that he doesn't really know what they are, but he's getting used to that. He doesn't know what anything is here, even less than he normally does. All the monsters at home are human.

He reaches the mouth of the alley just as one of the creatures catches fire – and doesn't seem to mind. There's someone in there, on the other side, but Will can't see who or where. It makes him hesitate a moment, because he doesn't want any friendly fire scenarios happening, but with the way things are going it's either shoot or let whoever it is die for sure. He's not sure how these things are put together, so he's guessing when he takes aim and opens fire.