After the apocalypse, Carl can deliver a baby with the added pressure of being surrounded by zombies and certain death of his mom. Bieber would cut and run.
Minutes after delivering his baby sister, and faced with two options: Mom as a zombie or putting a bullet in her head, Carl chose the latter. WWBD (What would Bieber do)? Faint.
When he and Rick discover Lil' Asskicker is missing, they mourn. That's more courageous than hiding pain and feelings like Bieber seems prone to doing.
It's not every teen who can responsibly use a firearm, but here Carl is again ready to clear a room of zombies. Would Bieber sing his way out of a jam?
Everyone says to leave Bieber alone, he's just a kid. Carl's just a kid, but he's fighting for his life every day. Only one would last in the apocalypse, and it isn't the Biebs.
