Back to the current episode, I found the fact that Melissandre's Spirit of Light cult was spreading to the rebels incredibly disturbing.

It's not like they're worshiping the old gods like the wildings, or something like that. The audience (and the salty smuggler, Brienne and Catelyn) have witnessed the results of following such beliefs, and it is quite plainly demon worship.

Praying to the old gods will never turn a fetus into a shadow demon, the act of doing so also damaging a man's life force at the same time, as she told Stannis why she wouldn't give him another one. Not to mention all the brainwashing, making it obvious that she chose Stannis not because he was a strong fighter, or a righteous man, or even a good leader, but because he was the easiest to manipulate.