Cobb: A clean dive? Perhaps. Mal, what are you doing here?
Mal: I thought you might be missing me.
Cobb: You know that I am, but I can't trust you anymore.
Mal: So what.
Mal begins as the epitome of the femme fatale. Cobb's inability to trust Mal stems not from her literal betrayal of him in the dream world, but rather from her betraying him and their children by ending her life.
Saito: He [Nash] sold you out. He came to me begging for his life. So, I offer you the satisfaction.
[Cobb is handed a gun.]
Cobb: That's not the way I deal with things… What will you do with him?
Saito: Nothing. But I can't speak for Cobal Engineering.
The first overt act of betrayal in the real world is Nash selling out Cobb and Arthur. He knows Cobal will be after them and hoped to get in with Saito. Unfortunately, Saito is a man who values honor and, as he's about to hire Cobb, isn't going to put up with Nash's backstabbing.
Arthur: So you knew about these risks, and you didn't tell us.
Cobb: There weren't meant to be any risks. I didn't know we'd be dealing with a load of gunfire.
Arthur: You had no right.
Yusuf: Besides, he said he'd done it before.
Arthur: Oh, he'd done it before. What, with Mal? Because that worked so good?
Cobb: That has nothing to do with it. I did what I had to do to get back to my children.
Cobb and Yusuf have just done what Saito was trying to avoid by ditching Nash. They've betrayed the group by withholding information for their own benefit. Look at how Cobb answers Arthur's critique, though. He says he "did what he had to do"… and what he "had to do" resulted in Mal's suicide. Now he's once again trying to get back to his kids.
Cobb: I went deep into the recess of her mind and found that secret place. And I broke in. And I planted an idea: a simple little idea that would change everything. That her world wasn't real.
Mal: That death was the only escape.
This act poses the biggest question of the film: did Cobb betray his wife? He was trying to save them but it ended up causing her suicide. Maybe Cobb's subconscious (in the form of projection Mal) is betraying him because it was he that first betrayed her.