I'm debating who 3XK is going to kill. I think his escape is going to come back and bite Castle in the finale by someone close to the team dying at his hands. I mean, why set it up if there aren't consequences?
My top candidates are Gina or Josh. The loss of someone the two main characters love could create another barricade in the Castle/Beckett relationship keeping the will they/won't they dynamic around for another season as Rick or Kate grieve. I figure Ryan's fiancee is another option. Or if the writers are feeling particularly daring they may kill off one of the lesser main characters like Montgomery or Lanie.
But right now, I'm leaning towards Josh.