He had a stronger connection with Aquagirl so it makes more sense. And the reason he went bad isn’t the point, the fact that he did is the shocking part.
That said, the line “blood is thinker than seawater” was a little ridiculous.
I think a reason why a character does something kinda is the point. Otherwise, it's just animated pro-wrestling. The Rock is a face one week, and the next, for no reason other than it's shocking, he's a heel. The question posed by his "heel turn" is "what could cause him to do such a thing?"
And apparently the answer is: "because a chick who dumped him got cacked. Oh, and also, because his real dad is evil."
For the love of Space God, I really hope they go into this more. But jesus, there are so many characters on this show now. Plus the characters they haven't even gotten back to showing yet.