At the end of The Darkness, Jackie succumbs to The Darkness by killing Uncle Paulie and has his soul possessed entirely by it. Game's over, Jackie gets his revenge but loses his soul a la Faust. So how exactly does he regain control over his body in between the space of time between the two games? We know that he sought the help of Johnny, but that isn't exactly something The Darkness is going to want to do on its own, meaning Jackie somehow gained autonomy and went for help himself.
And if the Darkness does control Jackie's body when he "dies", why bother relinquishing control whatsoever when it's clear Jackie has no intent of submitting?