I found it odd several characters had a double motive, or an eerie "I might be the bad guy" moments.
David:
Obviously the big one. Spikes charlies drink with the black substance, compromises the safety of the ship with the vessel (which was still on the ship the whole time), keeps opening doors and entrances (in the derelict) against the suggestions of the other staff (oops I opened the door that has the vases), attempts to awkwardly sedate and then cryogen freeze Shaw even though she is obviously compromised as well. Then does an about face, somehow survives the jockey ship crashing, and goes on a helpful space adventure with Shaw. He also read her dreams while she slept, which is a little bit on the unnerving side.
Vickers:
One that confused me. She obviously has the "corporate wench" attitude through the whole film. Yet she is shown having a connection with their mission (her teary eyed emotion as she watched the experiment on the space jockey head) is resolute about preventing contagion (so she knows little about Weyland/Davids interactions) yet still manages to seem like she is hiding behind some guise of deception the whole film until she is smashed in a horrible boring fashion.
Janek:
Seems very reasonable, until the two lost crew members find a blip abnormality in the system. All the while showing off the massive pile of dead bodies that no one else addresses. The captain even pretends that he can't see their feeds and tells em to sleep well. Wouldn't contact with say, 20 more bodies, be worth exploring and looking at when the staff returned after the storm? Seems like no one was watching the feeds even though they were reporting massive finds and completely (and stupidly) lost. For that matter, why would it be in Vickers interests to go have sex instead of watching the feeds when she is all about "making people do their jobs". So shes a tough as nails leader, until a strong black man wants to give her a poke, then she throws all caution to the wind? In the end Janek is an overall good guy, who sacrifices for the greater good, yet allows two people to die while he goes on a sexual escapade.
