(spoiler) In hindsight, Rusty probably absolutely hated us when we first met him.

I mean, think about Operation Wallclimber from his perspective - he's a spy for the RLF, forced to kill his comrades to keep his cover. Meanwhile, his ally is a mercenary who's happily slaughtering his friends for money and experiencing no apparent moral conflict over it. In all likelihood, the only reason he's nice to you at first is because that's part of his job as a spy - by being a likeable, friendly person, he can make social connections that can later be used to the RLF's benefit.

With this in mind, his actions during Unknown Territory Survey make a bit more sense - he's already betrayed countless friends for the greater good, what's the life of one amoral mercenary worth compared to that?