In the movie 'Vanishing on 7th Street' we see the initial power outage and how different persons reacted to that. After these initial scenes the movie jumps multiple days forward. In the next scene we see Luke going through the streets and entering cars. A man sees him and as his light is failing he begs for help and to get into the car. But Luke didn't let him in and the man is swallowed by the shadows. Why didn't Luke open the car door, let the man in and share his lights?
My feeling was that the man came on way too strong. He tried to barge his way into the car (prompting Luke to lock it), swore loudly and just frankly came across as a massive liability. He stated that he wanted one of Luke's lights (which were roped together and not easy to split up) but he didn't offer any positive reasons why Luke should risk his own neck for a complete stranger, one who might simply take all of his lights and leave him for dead. It probably didn't help that the guy was unkempt and poorly dressed
On top of that, Luke is scared, alone, can see the shadows moving in the distance and doesn't know who to trust or who's responsible. He most likely felt that the most sensible option was to keep himself safe first, then worry about others later.