Mostly common for villains, or people, who doing or about to do bad things on purpose for personal satisfaction.

Also known and a facial expression.

An "Evil Grin" is more subtle than a outright "Evil Laugh" but is still fairly exaggerated so as to make sure a viewer can not mistake it for a normal smile.
"He smiled at us, but the smile wasn't friendly; more amused, like Oh boy, more people to torture!"
- Percy Jackson when he first catches sight of Geryon (evil grin)
by someoneniemand September 18, 2013
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the face reserved for coitus when you know you are gonna cum real soon and have given up on waiting for your partner to climax.
"I knew I couldn't wait any longer so I put on my evil sex grin and came first. It was awesome"
by veeds1 September 6, 2009
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