Assuming it's inescapable—I would be very depressed about the obliteration of intelligent life, including my own.

As quickly as possible, I would try to get someone alone, torture and kill them. Nothing fancy, but I could not pass up the chance. Not while aware that I was going to die without ever knowing what it felt like to take a life, and especially since I would have relatively little to lose.

After that—I don't want to die alone. I'm going to be honest here and say I'd spend my remaining time with the people I love. I'm not religious but I might end up praying with my family at some point—there are no atheists in foxholes. And I'd want to die holding someone close. #cuddlymurderer At the present moment it would probably be my ex, since I'm single xD