Karen took Anthony's clues quite easily.  He was usually a bit better at making friends than she.  Usually Karen preferred to just kill and not risk getting hurt, but she knew that Anthony would know when or when not to get rid of Markus.  In this she trusted his judgment and skill.  However, when Markus looked back at her and saw the blank emptiness in her eyes it was obvious that if he put one toe over the line she would easily kill him without a moment's regret.