True, I guess he could inherit now, but he would have to make a claim if anyone disputed him, and he doesn’t seem to have any interest in that.
But Kevan and Lancel Lannister were killed, too. Jamie appears to be the only Lannister left who can inherit Casterly Rock.
I wonder if he’s feeling like he’s repeating the past where he killed Aerys II to keep the king from “burning them all”. Now it’s his twin sister who seems to have gone completely crazy.