Honestly Karkat is mostly just confused about why Julie is suddenly an exception to their kill everyone plan. (The reason is that Cuthbert thinks she's hot.) But yeah, he'll come around.

OTOH, Julie would probably be able to recognize Karkat as that kid who isn't in her district but spends more time in the Twelve suits than in his own District, so that might encourage her to join up with them when Cuthbert offers?