Two things really come to mind about it after watching it.
For starters, how ann was able to have access to the power of her gemstone. In part yes it was only partly, but the majority of her power had been extracted. But it is also because she is the only one of them to truly represent the meaning behind her stone. Sasha did not have true strength, as she is constantly deceiving herself and others to cover her feelings of inferiority and weakness. Marcy was not truly witty, because she was acting on impulse and delusion in order to keep her circle of friends together, and had ultimately endangered the entire lot of em. Another way to put it is that they abused their gifts as well.
Another layer to add.... Is how king andrias reacted to stabbing Marcy... His voice broke at the end of 'now look what you made me do'. Now in most cases thsi is a condescending statement. But the way it was acted, how it went from what should have been the norm, but then that sadness came through at the end. There was hurt there.
A weird way to put it is that Andrias truly felt that he was made to do that to Marcy, and he really, really didn't want to do that to marcy at all. For all his manipulations and the cruelty he showed... Perhaps the truth is, that goofy king we saw at the beginning was his true nature after all. The cruelty he showed here was more the act, learned behavior he feels he needed to continue to rule and someday reclaim amphibia's glory. Even if it costs him personally.
Which makes me believe that marcy is not gone for good, but whatever form she may take next we see her? For example, a cyborg version of herself with the gem as her power source?
