There are many motivations for draining others. Some of them include: Food (classic-style vampires). Enhancing their power. Satisfying a blood-consumption fetish, Horror Hunger or other psychological addiction.
Prior to a postmortem, the female alien awakens and drains the life force from a guard. She escapes the facility and drains other humans of their life force. The two male vampires awaken and violently attempt escape, but are apparently destroyed by grenades thrown by another guard.