When Private Jessup is slowly being murdered, David is being held back from saving him by his own group, who also wants to save him. There's no indication that it's too dangerous to stand up to the nutters. Regardless, no one would just stand there. They just needed to stay out of the way so Jessup could get thrown out, and there was no other way to show it without people acting in a way that doesn't make sense.

Scene in question. Immediately in the beginning the group of survivors push David & co away from Jessup, and this is before the scene truly escalates. There is every reason to believe a scared group of people led by a religious fanatic would be violent against (percieved) outsiders; in fact they are in that moment brutally murdering one.

Also when William Sadler's characters turns all those lights on when the monster gets in?? I've seen this movie multiple times and I can't divine any reason he would a. think that's a good idea. b. even feel the impulse to do that. It just has to happen to increase the danger and panic. And I guess so that guy can get set on fire.

Sadler and his buddy set up those light so that they, in case of an emergency, could light the store. Presumably to better fight whichever threat. It just backfired on them when the bugs were attracted to the light.