Even though things do look bad right now, Cassandra is not evil. Her friends are not and never will be her enemies. Anyone who thinks she's corrupt, manipulative, vengeful, power-hungry, malevolent, bitter, sinister, traitorous, sadistic, treacherous, or merciless is going to look like a TOTAL CLOWN when Season 3 happens.
As theory, i like to think that cassandra is like eddie brock of the spectacular spiderman, in that both had a hard past in that they lost their parents and should work so hard to could lives without a lovely parents that care them. And also, both were one of the best friends of rapunzel and peter parker, to the point of see them like sisters/brothers.
But as cassandra and eddie could see that rapunzel and peter had and have better lives than them, this caused that the formers developed deep down a grudge and resentment toward the laters, For example: in the Challenge of the brave, cassandra showed a lot disdain that rapunzel took the spotlight and leave the former like her shadow, showing a hint that cassandra resents the success of rapunzel, at least deep down, and eddie in several times in the spectacular spiderman, he showed envy toward peter by had uncles and good friends that care them, while he was alone without anyone else. Nevertheless, for a good time, cassandra and eddie could ignore this dark emotions and be good friends with rapunzel and peter.
But tragically would occurs some events that would trigger this inner demons: Like in Rapunzel and the great tree, in that rapunzel several times she ignored and dismissed the advises and concern of cassandra, making her feel like a no one, and to get worse, because rapunzel ignored the advise of cassandra of use the demanitus's spear to defeat hector and instead used the moon spell, the arm of cassandra was severely burned and this caused that the resentments of cassandra became more obvious toward rapunzel and once that cassandra learned about "her destiny" in the portals, she finally chose betrays rapunzel by get fed up by be her shadow always. In the case of eddie brock, he finally ends his friendship with peter when apparently the later, took photos of the symbiote being stolen by spiderman and black cat in the laboratory of kurt connors, and peter don't even try to call the police, making think to eddie that peter was a false friend and a "user", and to get worse, by the lose of the symbiote, kurt lost a lot incomes, causing that he had to fired eddie, leaving to the later without any incomes to lives and pay his university, causing that eddie blamed peter for his disgraces. And to insult to injury, when spiderman would plans to destroy the symbiote using a cold regrigeration device in the laboratory, eddie saw the attempt of spiderman to destroy the symbiote and in spite to beg the hero to not do this to try to retrieve his job, spiderman do this anyway and leave eddie angry with him and he though that spiderman was even worse than peter, but this outrage caused that the symbiote were attracted by eddie and once attached, it revealed eddie that peter and spiderman were the same person, making that both plans their revenge against spiderman.
I think this theory, because i think that is very dark that cassandra was always a false ally that since beginning was plotting against rapunzel during all the series, as cassandra should have a very cold heart to planned a betrayal against rapunzel without remorse, as if all the moments that both had in the series means nothing to cassandra. Besides, would be more interesting and deep that cassandra had a hidden resentment deep down toward rapunzel than be a wicked traitorous villain that don't have qualms in hurts to everyone., as this even could make that cassandra can be redeemed if rapunzel managed to reconcile with her to make her leave the opal to finally put an end to the black rocks.
PS. Regarding to the questions about why cassandra wasn't in the wedding of rapunzel, if my theory is true, maybe cassandra has so shame that she simply chose lives in exile by all the troubles that she caused, as others pointed that maybe cassandra would dies.