I believe they probably do believe in the Slender Man and it speaks to their unstable mind, but since they had moments of hesitation in stabbing her, it's clear they had some sort of conscience. Besides, their stories aren't consistent. They say they wanted to prove their loyalty to the Slender Man and run off to live in his mansion, but then they also say the Slender Man threatened their family. So which is it? But 19 times? That's overkill.
No need to blame on the internet.. two girls plan on how to kill someone since Feb. Two 12 year old sociopaths rationalize their sociopathic behavior. Both are trying to get out of attempted murder. they should be charge as adult or put in the hospital for life.
Just because they hesitated doesn't mean they had a conscience. They stabbed the girl 19 times... that's not someone with a conscience. Whatever their story is, it still comes down to the fact that they attempted murder. They would have succeeded had the girl not dragged herself out of the woods and found help.