First of all, good luck with the manga.
Q1: Probably start killing people that I secretly never liked, b/c why not.
Q2: I imagine most people will panic and start doing all types of crazy shit.
Q3: idk, pray to God or try to get other people to believe it's all the wrath of God or w/e religious types do.
Again good luck to you with the writing. These things are just my opinions.