"Ah, but a Lannister always pays his debts, dear brother. Did you not say how you would take revenge for that lowborn gold-digging whore? How you poisoned my son?! Did I not save you from the headsman only for you to spit on my face and kill our father?!" Jaime badly wanted to draw his sword and lop off Tyrion's head. Something stayed his hand, though. He couldn't do it. Tommen did not need a kinslayer for a Lord Commander or an uncle.