What a prick....his salvation lies in the hanRAB of the Alien and he knocks him out.....why?...have I missed something

....The only thing I can think of was that the alien was going to leave him behind....but if this was the case the dialog did`t make this scenario clear.....The alien just said it would
take 3yrs....Not "Ill be back in 3yrs....although after the shoot out the alien made it clear he`d return in 3yrs..