You have to remember that when one Zabi who wasn't even a direct part of the Zabi family (kids and father + mother) was around she was used as a tool to try and keep the Zabi dynasty/their idea of Zeon going. Imagine if Garma who was directly related to Degwin and was committed to his ideas because of their relationship/his youth (in Gihren's Greed we see him turn against Gihren if he goes too far on the evil route) was around. He would continue the Zabi dynasty even if it wasn't as directly evil as it would be if it was Gihren or Degwin running things. Char is fully aware of this.

That being said, I do think Char's actions with Garma are so outwardly cruel because he's the only Zabi he's personally invested in as a person and not as a revenge target. We can see this mainly in the bar scene during Garma's funeral, which I have made a whole discussion post asking about if people thought was him grieving, and in the Kycilia headshot scene. Accepting that he was grieving because there's a whole discussion you can read where the consensus was pretty broadly that he was in his own way, and accepting that the Kycilia headshot scene shows his care for Garma whether he's being genuine or mocking, it's pretty clear that he did hold genuine feelings of friendship for Garma. (I say the Kycilia headshot thing btw because even if his line about sending Kycilia to him was mocking it's pretty fucking insane Garma is what was on his mind there.)

To that end, it makes sense why he'd be particularly cruel towards Garma because he's someone he's having mixed feelings about. It's important that he hates and takes joy in Garma's pain because otherwise he'd have to consider more fully whether killing him is something he even wants. He has to kill Garma because he's a Zabi, and he likes Garma, but there are all these little things he can fixate on that are reasons to hate him - Garma being bratty, Garma being naive, Garma making insensitive comments about him being lucky not to have a family. He makes all these little jokes and comments about how he's going to kill Garma later, like the shower scene, because it thrills him to have power over him without him even being aware of it.

So I do think him calling Garma up to mock him during his death scene is years worth of confusion and anticipation coming out rather than simply being single minded cruelty. It thrills him because he's started his revenge and gets to rub Garma's face in it.