Complex sentence: Realizing that he joined up out of a sense of obligation to his family, Sid DORs, and then leaves the base without saying goodbye, so Zack and Paula go out to look for him.
Split into simpler sentences: Realizing that he joined up out of a sense of obligation to his family, Sid DORs. Sid then leaves the base without saying goodbye, and Zack and Paula go out to look for him.