In any "instant destruction of Earth" scenario, I'd believe I would probably say my goodbyes to everyone, grab my possessions, and head to a place that'll house my last few days.
I can imagine it. The world is erupting in chaos. People are pillaging, raping, killing, and doing all other kinds of dark things. All the while, I'll just try to find a nice coastal town, find an empty house near the beach... possibly one with internet, if it's still running, and just live until the end arrives.
Naturally, weapons included.
I wouldn't want anyone to ruin my relaxing end as I see the giant rock zipping towards the Earth... although that really depends if I can even see said "giant rock."
In that situation, I can imagine I'll give into some dark urges. Like wanting said settlement to became a complete ghost town. Even if there were groups of people high off their rockers, or making some kind of slapping, pounding meat mountain orgy... I'd like to get rid of them. I mean, fuck em', I'd like to be a little selfish.
...Also by "get rid of them" I mean that it can range from blood to run away with only their clothes. Hopefully the latter, if I'm in a good mood.
Although, if it was the whole "Heaven is going to destroy the Earth" scenario, I could imagine I would be a little more passive. I mean, I am religious (not the crazy kind, thankfully), I'd probably think that an invasion of Angels would be an amazing sight.
But yeah, getting back to the main topic.
Giant rock hitting the Earth, killing us all, I'd think that I would try to relax.