User blog comment:HalointheSkye/Volume 3 Chapter 9 - PvP (IN WHICH A LOT OF SH*T IS HAPPENING)/@comment-25405817-20160118033856/@comment-16241789-20160118080058

My guess is that Mercury was in maintenance to take control of the camera feeds in order to prevent them from cutting. When Ruby came snooping, he stopped her to keep her from potentially discovering something. Why did he delay her instead of just fighting her off? Well, if she had run, she would've raised the alarm, which would've resulted in security officers being sent to investigate maintenance, which would've turned something up for sure. So he kept her busy until the chaos set in. When Ruby saw Penny and collapsed in tears, Mercury retreated to keep from arousing too much suspicion, confident that Ruby would be too traumatized to actually do much. His job was basically to keep the maintenance area secure and keep people from snooping so that the feeds wouldn't be cut, allowing Cinder to make her speech.