Why Sirius thinks it was Remus:
"Don't do it Sirius, don't-" too late. The wand had appeared in the man's hand, a bright flash of light shot out of the end, and the man fell dead before his friends warning had even registered.
His face was blank and cold as he stowed away his wand, then turned to face Remus and said, "You going to tell on me?"
It was an old joke, that used to be funny. Now as the two stood over a body, it fell flat like dead air.
Remus swallowed, his eyes still flickering between the corpse and back when Sirius snapped, "he was a Death Eater, and you were trying to get him to surrender. Merlin sometimes I wonder about who's side you're on!" He was already turning away without a second thought. It had been a long night, he felt exhausted and wanted to go and check on Peter before he really went to bed. Next weekend was his birthday after all, he thought he deserved a bit of a break. It took a few steps to register that Remus hadn't called him back, or corrected him for his little snap, or done anything really. He turned on the spot to disappear, and felt a sudden bolting chill of fear when he saw the look of unease he left behind on someone he once would have never considered…but he was being paranoid surely. Nothing was going to happen.