[Florian the Fool]

I like the sound of Arya as a Killer Queen. With gunpowder, guillotine, dynamite or a laser beam, she'd be guaranteed to blow my mind. Anytime.

Sorry, that's just a little classical reference for those able to appreciate it.

Arya might not accept a contract to kill Jon, but she might accept a contract to kill the Lord Commander of the Night's Watch if she doesn't realize who that is.

Plus, I don't know how much we have been told about how the Faceless Men operate. It may not be a question of picking and choosing contracts - it may simply be that you are sent to assassinate someone or other and you have to obey your orders. Obedience certainly seems to be part of their training.