Rin is Rin and Dark Archer's thought process makes him either a dick or senseless
Well, he is sort of a dick to everyone who isn't Rin, so that means I'm doing my job.
Dark Archer's MO is pretty obvious. Rin>>>> Before everyone else. Unless Ilya shows up. Then it would be Rin>Ilya>>>>> Everyone else.
Unless its something that would effect/hurt Rin. Like if Sakura shows up and is in danger? D. Archer will save her despite the fact he hates Sakura with pretty much every fiber of his being. However he'll save her because he knows that will make Rin happy.
Like if Rin wanted a threesome with Lancer, she could talk him into doing it. Of course he'd be a giant douche to Lancer the entire time, but it's what Rin wants.