Rapunzel is the worst friend to have because she's not inherently malevolent. She's selfish and ignorant of other people's emotions. Traits all sociopaths possess. Varian and Cassandra chose to help her out of their own goodness, traits that Rapunzel exploited for her own benefit while completely unaware of how much it was costing them to care for her.
That's why they couldn't tell her how much of a jerk she is to her face, they felt guilty due to her upbringing so they kept helping her, all while piling up resentment every time she let them down.
Since they were guilt tripped into accepting she wasn't at fault, the anger built up until they exploded and drove them both to insanity. Meanwhile Rapunzel went on her merry way to pursue whatever picks her fancy.
To reach her destiny, she will gladly step on the corpses of her followers, using them to build a stairway to her dream. She will offer her friends and lover to unspeakable horrors on a whim, all while keeping that smile on her face.