This is how The Symbiote betrays the Dazzlings goes in My Little Pony: Equestria Girls: The Symbiote Siren.
[Adagio helps the Symbiote get ready]
Adagio Dazzle: Well, it's nearly our time thanks to you.
The Symbiote: No. Actually, It's nearly my time.
Adagio Dazzle: What?
Aria Blaze: Huh?
Sonata Dusk: Say what?
The Symbiote: You three helped me grow strong, you've provided the means by which I can use to defeat Twilight and get her magic too. And now you're no longer of any use to me.
Adagio Dazzle: But you said you'd help us be adored and defeat the Rainbooms.
Aria Blaze: Why would you betray us?
Sonata Dusk: Yeah.
The Symbiote: I lied, as I have from the very beginning.
[The Symbiote then steals the power the Dazzlings collected into him]
Adagio Dazzle: You never planned to kill Twilight and get her on the Dark Side.
The Symbiote: Not with you, no.
[The Symbiote shuts them in a closet then goes off to continue his plan]
The Symbiote: I'm going to need you girls out of my way if I don't want you or those Rainbooms interfering with my plans.