Assassins are either legal, or illegal. There were some, legal ones, who work for the government and are able to get into any place they want. Then there are illegal ones, ones who went rogue and began building their own groups to take out political leaders and famous citizens for their own entertainment or greeds. Minho was a legal assassin. Working straight from the government to keep citizens safe. That meant keeping his guard up and always having a keen eye for suspicious behavior. Meeting Jisung was just a bonus to his job.