Why do Born Agains tend to act more obnoxious than, say, people who were born Christian?
That's actually kinda easy to explain.
They have more to "repent" for. And by golly, they're going to go the whole nine yards because they've got a lot of making up to do.
It's like going on a fad diet, say, one where you eat only melons, and angrily insisting that everyone in your house eat only melons, too.
And by "house", I mean "planet".