>>149919724
To make him suffer. Sugar admitted she liked seeing Finn cry and contrary to your statement, she did plan to break them up to see Finn cry again, something she did in Incendium too. Muto is bad but mostly because he's a spineless simp to Sugar and Allegri's antics but yeah, he joined in on kicking Finn in the dirt to fit in with Sugar's hatred for him. Remember Sugar only fucked off from AT because she got her own show and therefore got bored with torturing Finn since she got a whole new toy to play with. If Steven wasn't an amalgamation self-insert of Sugar and her brother, Muto & Sugar would of had him commit suicide or get stuck in bubble hell in SU: Future.
>Allegri
Look at her writing in the new Fionna & Cake shpw. She went out of her way to show her disdsin for Finn in multiple dimensions and wished Fiona could erase and replace him entirely. Again, as many pointed out it's because she's a toxic insert incel who's a cancer on everything she gets her hands on.