It seems that these kinds of questions -"If blank happend, what knife would you carry?"- are getting more and more silly. Nevertheless, I will attempt to answer.
If armegedon occured and aliens invaded the earth, I would want to carry a strait razor so I could slit my wrists. I would figure that; a) I have gone completely insane and am so far removed from reality that treatment would ultimately fail and my best option is suicide, or b) The world really is coming to an end and I would rather die than see such horrible suffering on a global scale.