Complex sentence: During the ensuing fight with Factionless members aboard the train, Caleb makes his first kill, and, horrified by what he has done, neglects to help Tris as she is attacked by one of the Factionless.
Split into simpler sentences: During the ensuing fight with Factionless aboard the train, Caleb makes his first kill, and was horrified by what he has done. He even neglects to help Tris as she was attacked by one of the Factionless.