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?
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.
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.