Sly could be one option. Shrewd and sly could be another one in certain instances.
The kid didn't seem the least bit scared, nervous, or worried. At the most, you could say what do you mean, “get rid of her”?' Harry Potter flashed a knowing smile, like a sly child. That smile made Montalbano’s blood run cold. Want to bet the guy quite sincerely considered the murder and butchery of Ninetta’s body just a game? A boyish prank? Want to bet that his form of homicidal madness was a kind of unconscious infantile cruelty? Like cutting off the tails of lizards?[/i]
[i]Sampson’s consistent errors in judgment brought about his physical death at the hands of a shrewd and sly woman named Delilah, rather than a victorious and godly life of being in service to God on this earth.
Examples from the Web.
There are other words to describe it, as someone described as a "shady character" (it rings someone as deceitful, suspicious, deceitful and dangerous).
And there are other words and expressions to express that, let´s wait for further comments on this one.