Question: The scientists of the genetic research lab were first attacked by the Hellknight monster, which was actually the human test subject. Since after the incident the research lab was quarantined and the first Hellknight could have gone nowhere, how is it that the Marine troops didn't encounter the first monster even once when they entered the genetic research lab?
Question: In the end of the movie, why did Reaper grab the BFG? I could see the stock of the gun sticking out from behind him. It had no more ammo left, so what's the point?
Answer: From what I know, that gun is now basically a working prototype, it would a nice thing to have. Also, it could be adapted to the RRTS (Rapid Response Tactical Squad) needs. Or just as likely, the UAC wanted it back.
Question: What exactly happened to Reaper's parents? I didn't get much from the flashback, and it's almost never mentioned.
Answer: They died in a cave in, in the archaeological dig area. Or at least that's what I got.
Answer: This is explained by the fact that it was likely hiding, waiting to see what the RRTS would do. After all, the human test subject still had its personality, as far as is known. It would've waited to discover their weaknesses, stalking them like a predatory animal stalks its prey.