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They simply didn't want to risk ending up in the firing-line themselves - when it comes down to a literal life-or-death situation, you'd be surprised (and upset) at just what people would be willing to do or not do in the name of self-preservation, even if it means letting another die. Consider: Richard's been absent for a while by that point (and the film repeatedly goes out of its way to demonstrate the "out of sight, out of mind" mentality held by most members of the beach community); Sal has just publicly revealed that Richard had broken the rules by copying Daffy's map; the marijuana farmers with big scary guns have made it painfully clear that either Richard dies by Sal's hand or else they're all dead, and none of them have any way of knowing that the gun the head farmer has given Sal isn't loaded. It's not that they wanted Richard dead, exactly, but more that given their knowledge of the situation, it seemed the safest solution for the rest of them 3- helping Richard at that point would effectively be tantamount to saying "you know what? You can just go ahead and put a bullet in my brain too because I'm perfectly fine with more people turning up here". Besides, there's very little any of them could have realistically done to help: the farmers had all the exits covered and were spaced out amongst the crowd. By the time Sal pulls the trigger, most of the community had completely privately probably just accepted that Richard was a dead man walking no matter what, and that since everything had become so utterly messed up, they were just going to sneak off the island the first opportunity they had once the farmers were gone - hence why no one except Bugs, Sal and Richard hesitates to run once it's revealed the gun wasn't loaded; they'd already made up their minds that they were going to leave, it was just a lucky bonus for all of them that Richard was going to be able to escape too.