Inertia
The world is ending, and no, he isn't being dramatic for once in his life. The world is really, seriously, ending and all he can do is watch it crumble. It doesn't matter how fast he runs or how far away or where he ends up – this is still happening and nobody can stop it because all the heroes are dead and who's he kidding when he calls himself one. He's not a hero. He's just a kid – a really, really messed up kid in a world gone wrong who's watching his everything get torn up and burned down and ruined.
"Bart!"
His father calling his name snaps him out of it, the momentary stand-still he had slipped into. His mother's already gone, and his father refuses to leave her body, and he's not going to get out of this alive, and, Bart realizes, this is the last time he's ever going to see his father alive.
He stares at his father, a halo of fire and light surrounding him. He wants to beg, to plead, for his dad to just get up, and maybe they can make it out of this together, but it's too late and there's no time and –
"Bart!"
"Dad..."
Because even if he knows what's about to happen, even if he can't do anything about it, some part of him wishes and hopes and prays and wants nothing more than this all to be a bad dream. But it's not a bad dream, and this is all very real and there's no stopping it and this is the last thing his father's ever going to say to him, and so help him, he'll live by whatever it is.
"Bart, run!"
And he does.
And he never looks back.