The only characters from Aliens who had clear motivation were Burke and Ripley, and maybe Hudson. Bishop's were deliberately left vague so we'd wonder if he was going to betray Ripley. The rest of the marines presumably just wanted to get in and out, but this was never made clear. However, if we are going to complain about inexperienced and/or stupid characters in Prometheus, what about Lt. Gorman? Why would Weyland send a completely inexperienced commander on a mission that potentially dangerous? You could defend their choice by saying they didn't take the treat that seriously - except Burke and presumably the rest of the company knew there were aliens on the planet. Why would they allow the mission to be commanded by someone who could potentially get them all, including Burke, killed? I can tell you why - because the inexperienced commander allowed the plot of the film to get going, and allowed Ripley to have her hero moment where she takes charge and gets the marines rescued. I don't want to turn this into a thread about another film. Just pointing out that you can nitpick any film to death, and the complaints I am seeing leveled at Prometheus are just as applicable to other films.