If you research about the term "Assassin"... they are basically hitman for hire. You pay them to do your dirty work, they'll do it without a second question, provided you pay them properly.
Now, the word Misthios, is a Greek term, for the english word "Mercenary". These people are also paid killers, who can be hired. For me basically, Agent 47 and Assassins are almost the same, except Agent 47 is way more stealthier than Alexios or Kassandra.
Therefore, my conclusion... Mercenary and Hitman are practically the same thing.