What would happen if a big asteroid was headed towards Earth and it was big enough to kill us all?

I don't know about everyone else, but I would focus on building a fall out shelter if there was time or try to negotiate with someone that had a fallout shelter to use theirs by binging food, water, flashlights that you wind up and never run out of battery life and being their slave. If the owner of the fall out shelter didn't deem me worthy, I would help him/her supply his/her fallout shelter, find him/her someone more worthy and protect the fallout shelter from the selfish people who try to sabotage the fallout shelter, because the goal would be for as many humans to survive as possible. Another option would be to help in an effort to send as many people in space as possible, weather it be bring materials necessary for fuel or guarding the space ship and other relevant facilities. Committing crimes and going insane would serve no purpose, only humanities survival and saving as many lives as possible would matter.