Jacks didn’t know why was he helping you during the Caraval, and if he was he wouldn’t tell you. And, if you didn’t know better, you would’ve believed that he cared about you (he did) in his cold, cold heart. After a while, you two parted ways. Still, when you were bargaining to save Donatella’s life, your experience with Jacks should’ve been enough to make you reconsider. But you said yes and the price was your blood.