Why did cassie leave the barracks where vosch had gathered all the adults

Because when Vosch called for everyone to gather together, that didn’t sit right with Mr. Sullivan and when suddenly Cassie noticed that someone was missing from their camp, Mr. Sullivan silently told her to go back and find him. That’s when soldiers reveal themselves a part of the 4th wave, and Cassie witnessed Vosch shoot her father for attempting to crawl away. And then her father silently told her to run before he collapsed.