Why did Cordier kill the man who brought him Mercaillou's body?

He had to kill him because when Rose went crazy on the black guy, she blurted out that Cordier had killed her husband.

As the previous said, she did panic a bit and blurt out that Cordier was the killer. As far as Mercailou bringing the body back...well, that was the home of the body. The rightful owner. If you watched the movie, they eluded to whites (French) receiving proper burials and the dysentery victims being thrown into the river. Actually, more than once. Before Mercailou was murdered, Cordier asked him for assistance in burying the body. He actually stuck around for his own murder and even rushed Cordier out of the house to do it. I don't believe Mercailou knew Cordier was in the house at all because he wasn't supposed to be. His home was with huguette, so I think Mercailou just brought the body back because in his mind, it was the right thing to do. Cordier knew he had to kill him when he was talking to her in the house cause there was all sorts of foreshadowing that he was being pulled in deeper from murder to murder. You got the sense he regretted what he was about to do, why he took so long getting dressed to go outside in the first place. At that point in the movie, his tone completely changed to a sense of hopelessness.