I feel like 99% of dumpees, myself included, are always shocked by how mean and cold the dumper becomes. Rationally thinking about it, someone that you spent a significant of time with can't forget you or just turn mean to someone they where with for such a long time overnight... people just don't work that way unless they are sociopaths (and I doubt that 99% of dumpers are sociopaths). I'm fairly certain, that unless you were the worst partner ever, it's a defense mechanism. Much like when teenagers start to 'hate' their parents - they don't really hate them, but its a way of emotionally detaching from the situation and the person.
So ex's aren't mean, cruel or cold.. they are just lying to themselves to detach from the situation. Come to think of it, I actually think that it's the ex's with the minimal reaction that act like nothing happened are the ones who are not effected by the break up.
But I guess knowing that my ex doesn't actually hate me doesn't make me feel any better