User blog comment:FairyRaven/Fairy Tail Chapter 471: Until the Battle is Done, Review and Commentary/@comment-1359322-20160201143636/@comment-7418318-20160201145313

Jacob's an assassin and he saw an enemy with his back turned to him, so he probably tried to do what cames with the territory and stab Acno in the back.

Lucky for him he had someone smart with him though. ^^