Pretty much everyone in that movie that died on screen committed some kind of fundamental "sin".
Gennaro's was cowardice: he abandons children in danger to save himself and go cower in the loo. He was also shamelessly greedy, but that was more Dennis's downfall. Admittedly Muldoon's "sin" of pride is very debatable, but he did think he could outsmart the raptors despite spending the rest of the movie telling everyone else how dangerous they are.
Mr. Arnold on the other hand didn't do anything wrong, which is likely why his death was off-screen. It was just used as a jump-scare to ratchet up the suspense as that scene wasn't about him, but about Ellie.
