A Face Of An Angel ,, A Mind Of A Demon..
Much of the time in keeping with Black and White Morality and Beauty Equals Goodness, we expect to be able to tell good and bad characters apart just by looking at them.
We assume someone’s good if his face looks innocent, compassionate, virtuous or honest while we assume the worst of someone who looks mean, untrustworthy, immoral or threatening.
Therefore it makes for an unsettling contrast or even a shocking revelation when a character looks benevolent on the outside but turns out to have a malicious personality.
On the one hand because he look just like you would expect a good character to look, he could try to inflict more harm by deceiving others about his real intentions.
On the other hand he could be completely upfront about being Evil and allow his enemies be spooked by the discrepancy between his appearance and his actions.
This is different from Beauty Is Bad or Evil Is Sexy because looking attractive and looking innocent or virtuous are not necessarily the same thing.
A Killer Rabbit is someone or something who looks weaker than he is but not necessarily more innocent.
A Devil in Sheep’s Clothing is someone who pretends to be nice but is actually Evil.
Being nice or mean isn’t the same as being good or evil which is why we have tropes like Affably Evil and Good Is Not Nice.
Very close to this is Wolf in Sheep’s Clothing, when someone disguises himself to look less threatening but who might be anti heroic or misunderstood rather than pure evil.
Another related trope is The Fake Cutie who likes to act cute but whose real personality is anything but.
This is the parent trope to Infant Terrible when applied to children since a common assumption is that Children Are Innocent.
Compare Divinely Appearing Demons when the inhabitants of hell superficially resemble creatures of heaven and contrast with Obviously Evil.
Which is when a bad character looks as evil on the outside as he is on the inside.
The inverse is Face of a Thug when a character looks evil on the outside but is either good or neutral on the inside.
Frequently combined with Light Is Not Good since light is usually equated with holiness as well as Pure Is Not Good since purity is usually equated with innocence.