He runs away.
The murderer has a burn mark on his face. His pops has a burn mark on his face. But his father can't be a murderer, so it has to be a coincidence or something, and his father is just pretending the burn mark doesn't exist because... because...
Because Sebastian is an idiot, probably, and can't figure it out even though Pops is acting like it's obvious. And apparently every accomplishment he's ever had was a lie too, and he was too dumb to notice, and maybe his pops is right and he really isn't worthy of being called his son. He can't win a debate contest that wasn't rigged and he can't save his pops from being accused of murder and he can't remain in this room one second longer without turning into an inchoate, sobbing mess.
The elevator ride is fifty stories, which really ought to be enough time to collect himself and absolutely isn't. He gets to the bottom floor and sprints out the doors and absolutely is not paying attention to anything around him.