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I would probably look elsewhere and try not to make direct eye contact because I don't want this guy to associate me with anything. I have a very photographic memory and I can place people with situations; good and bad. I perceive others to be the same way (even if they aren't) and thus try not to be noticeable so that others cannot associate me with a place, event, etc.
First of all, my reaction would be, how does this person know who this guy is? Then I would become very suspicious of the person sitting next to me. I may even decide to sit somewhere far away from the person in the lobby because I would be feeling very uncomfortable at this point, being next to someone who knows underworld characters. Next, I would feel very afraid and fearful for my life because how could a murder be walking around free, as if he never committed a single crime in his life? And finally, I would be disgusted and take all precautions to avoid the hit man.