The Evil Grin is a very common theme in fiction dealing with villains and is comparable in many ways to the equally famous "Evil Laugh". However, the "Evil Grin" is normally silent and is meant to show the audience that the character is evil and/or plotting something.
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.
Subtle GrinThe "Subtle" Grin is a type of "evil grin" reserved for particularly cunning and dangerous individuals who often manipulate those within their settings into thinking they are either good or neutral: yet when their back is turned or they are in seclusion they will display a small, sinister smile that shows the audience that they are not as innocent as the other characters think they are.
The "Subtle" Grin can also break the fourth wall when a villain suddenly turns to the camera and gives a small smirk to the audience - this was made especially famous in the original ending of the first Omen film.
Manic GrinWhile debatedly not a truly "evil" grin as much as a "completely insane" grin, the Manic Grin is shown when a story wants both the audience and the protagonists of the story to realize that the villain is completely out of touch with reality and most likely a Homicidal Maniac.
Much like the Demonic Grin, people confronted with the Manic Grin feel threatened and may well be in serious danger. However, unlike the "Demonic" Grin, the Manic Grin does not always automatically mean a villain is going to attack a hero (though they often do): sometimes a Manic Grin is simply used to show everyone how insane a character is and some of the more extreme examples of a "Hollywood madman" display a Manic Grin on a near constant basis.
Demonic GrinThe Demonic Grin is one that makes no attempt to mask the "evil" of the character in question, most importantly this grin is very visible to those involved in the story and often frightens them intensely.
Being at the receiving end of a "Demonic" Grin is a very unnerving position and it normally means the protagonist is in imminent danger from an attack or other misfortune as the villain does not even bother to hide his or her malicious intents: in some ways the "Demonic" Grin can be seen as a symbolism of the monster within being unleashed and like the Manic Grin it is also clear to the other characters involved in the story that the villain is now ready to inflict some pain on their unfortunate target(s).