Personally I doubt Jaimie would kill Cersei. There was a scene in the last series where Jaimie forcefully explained to his prisoner Edmure that all he cared about was his sister. She still might die though. Ayra maybe too predictable now after it's been made to look so easy for her. It might be at the hands of Olenna who wants revenge and is now allied with Daenerys via Varys.
If Cersei dies is Jaimie next in line? He is young enough to start a family and continue the dynasty.
He explained his incessant need to return to cersei (if I have to kill every last Tully myself) before he discovered that she'd essentially lost her marbles and the obvious shock on his face when he confronts her probably diminishes his desire to give her the pink torpedo