You shoot the zombies and kill them. That means less ammo, but you doubt the knife would have done much good against that many zombies.

You are close to the helicopter, but you know they would never leave Hank. You'll have to find a way to get down securely, but using the helicopter to get to a floor you don't know the status of could be risky. Anything could be behind those doors once they open, and the floor you are currently on is the only one you know is secure.