A Lannister always pays his debts

He blamed himself for Myrcella's death. He'd sent her to Dorne and those Sand Snakes had killed her in vengeance for Oberyn's death.

He blamed himself for Tommen's suicide as well. For if Tywin had been alive the whole situation with the sept and that whole fallout never would have happened. Cersei had even said as much many months ago, and he knew it was true.

He knew he had to make amends. After all, a Lannister always pays their debts, and he was still a Lannister no matter how much he wished at times not to be. He didn't know if it would even be possible, but he would make the best attempt he could to pay his.

Tyrion Lannister was going to bring his siblings back. Back from death.