I think its Scarlet because Scarlet went to check on Red, and it said he ran back tot his friends. In the video, it showed he was running, so he went to the room and killed Red, so he ran to his friends and told them that Red was dead so it was obviously him.
idc anymore, but i think that it was scarlet, because scarlet left before to kill shamrock as it shows the pills dna scan, and also scarlet was acting suspicious earlier on.