I agree with the person above me, though I'll also add that the comic itself contributes to this somewhat. Terezi undergoes something of a transformation in act 6 that doesn't feel especially earned, and mainly serves to infantilise Terezi in many ways. Not to say Terezi wasn't somewhat complex or vulnerable in the earlier acts, but it's like all her eccentricities and defense mechanisms disappeared off-screen for the sake of making her more melodramatic, and those very things are what contributed to her moral grayness, as they do for Vriska.