The description sucks, but...

Not every villain has to sound like one. Perhaps, instead of having a malevolent rasp, a gloating shrill, or a booming baritone, the villain's voice is instead light and low. They don't yell or intimidate, and might not even be impolite - instead they speak with a solf tone that seems unassuming, meek or even kind, with a soft chuckle and a sort of warm energy... hiding the menace within. The end result is something dissonant and creepy, a monster who might describe just how horribly he's going to mangle you, while speaking in a voice that's anything but monstrous.

See also Affably Evil, Dissonant Serenity, and compare Wicked Cultured. Note that, if and when they eventually drop this softspoken demeanor, then that's a surefire sign things are going to get bad.

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Examples:

• Kaa from Disney's Disney/The Jungle Book, as played by Sterling Holloway and Jim Cummings (both of which used similar voices for Winnie The Pooh), practically defines this, especially in the first movie where, despite having the gentle voice of an genial old man, is menacing enough to scare Bagheera. Later, he uses that soft quality in his voice to "convince" Mowgli to trust him.
• Sterling Holloway's good at this, he also plays a mysterious but malevolent television repairman in The Twilight Zone episode "What's In The Box," where he acts the role of the friendly old repairman who gives an exasperated man some extra perks in his cable, and then gets to watch, smile, and subtly gloat as his "product" ruins two lives, never dropping the unassuming facade.