The reason why Bee still cares about Cole despite her intentions with the cult with his blood is because she loves him more than a friend.Yes, I know it's really impossible in real life, but think about it- there's more reasons why she only wants the vial of his blood than having him be a human sacrifice like Samuel was- she could have a hidden crush on Cole, and why she ended up killing Allison in the first place. Her last scene after Cole rammed the car to her has some hints of this before she lost consciousness.
Bee's wish made true through the sacrifices was because she was either a target for the bullies as a kid, or mostly because she was raped or abused by family members.I mean, what else would Bee do the sacrifices for? She just wouldn't be strong and confident out of nowhere now, does she? This might be also why she really does care for Cole, because she didn't want him to be like her before she did the sacrifices.
Bee got introduced to the cult by her babysitter.This ties into the WMG above. Some years ago, Bee was like Cole — a meek little kid who got picked on. She may have also been being abused by her parents. Her babysitter did the same thing she does now, putting Bee to bed before sacrificing people, using Bee's blood to complete the ritual. However, the babysitter did really like her and have genuine empathy for her situation. So when Bee found out, rather than kill her, the babysitter offered her a chance to join the cult herself, and she said yes. Now, Bee's repeating the tricks of the trade she learned from the person who got her involved.
Bee was Grace in Ready or NotMuch in the same vein as the previous WM Gs, Bee used to be Grace, and after a night of being hunted, she turned to the dark side, and made a deal so she would never have to feel weak and helpless again. Looking younger was just icing on the cake.