Why? (spoiler)

As I see it, 'the guy' was expecting her to shoot McCoy, as they had suppossedly agreed before, but she then chooses her husband instead (she later says that to him) and kills the other.

Carol had to sleep with Benyon to ensure Doc's release from prison. She was afraid that Benyon would reveal all that to Doc. So she killed the guy. Doc realized it soon and thats why Doc slapped here several times after that.

She was supposed to shoot Doc, according to her deal with Benyon. That's why she was trying to talk Doc out of not going there at all (because she didn't really want to), but Doc had to hold up his end of the deal and all that.

So when she walks in and draws the pistol on Doc's back, Benyon doesn't care and expects her to kill Doc, as was their deal.

Instead she turns and kills Benyon, because she loves Doc AND because she doesn't want Benyon to say any more about their tryst, though essentially Doc already knows so it doesn't exactly conceal anything. Rather, it confirms what Doc has been unwilling to face; that she chose to sleep with the guy and agree to kill her husband for him.

Makes her betrayal of Doc much deeper (not only was it some sex, but "he got to her" as they say at one point and she had to really choose which man to off and which man to stay with.) But also makes her choice and return to Doc all that more poignant.

Hence the confused turning of the gun on Doc afterward. And Doc's prompt drawing his gun on her. That's when it all came together for him; when he could no longer deny why he got out and what it meant.

And that's when his treatment of her really started to go downhill (before they have their heart-to-heart in the dump and rebound towards their happy getaway).