Nash was the architect who designed the two dream environments we see at the start of the film; Saito's Castle/Dining Room and Saito's Apartment. It was his failure to correctly research the layout of the apartment (which was based on a real place, something that Cobb vehemently opposes) that resulted in Saito realising that he was still in a dream.
Fearing for his life after having been seen by Saito, Nash evidently approached Saito directly and offered to sell out his partners in return for being left alone. Saito, obeying the age-old maxim that you can't trust a traitor had his men beat Nash bloody and then offered to hand him over to Cobb to kill.
Cobb refused to shoot him, but neither did he offer to vouch for him (the guy's an incompetent and a traitor to boot) so he just stands by and watches as Saito's men drag him away. We learn from Saito that he's not going to be killed. He's probably just going to get another kicking before being thrown out into the street.
Nash, BEATEN BLOODY, sits on the far side, slumped against the window.
Beside him: SAITO. He nods politely at Cobb.
SAITO: He sold you out. Thought to come to me and bargain for his life…
Saito’s BODYGUARD offers Cobb a GUN.
SAITO: So I offer you the satisfaction.
COBB: That’s not how I deal with things.
SAITO: Would you work with him again?
Cobb shakes his head. Saito’s BODYGUARDS PULL Nash from the chopper.
Saito motions Cobb and Arthur to sit. The chopper RISES. Cobb watches
Nash DRAGGED across the pad.
COBB: What will you do to him?
SAITO: Nothing. But I can’t speak for your friends from Cobol Engineering.
INCEPTION: THE SHOOTING SCRIPT