I think it's rather the opposite. People can look perfectly fine on the outside, but be monstrous on the inside.

[She thinks of Eren, which really isn't fair because he cares so much - but his care for his people will make him do terribly things (from his perspective, he's already done them). But isn't she the same, only on the opposing side? But no - her people can't be judged by the same standards as others, not after what they've done to their world.]

...

Perhaps, some people really are nothing but monsters.