I wouldn't immediately assume it's insecurity. I used to be an atheist for a good decade and a bit. I remember making fun of Christians in high school. You get to be edgy and the Christians start to feel real bad because you're making fun of what is their "silly sky daddy" to you. But the extra fun part is they're conditioned to be nice in response so it's like you get to sit on a tall branch and drop things on their head and you don't have to take so much flak from them. And if they do act out, you can call them a hypocrite and insult their faith because it clearly doesn't work. It feels like a fun game you can't lose, until one day things get too real and you discover in horror what you'd been doing.