What can you call a person who is cunning but depicts themselves as innocent to others?
snake in the grass (dictionary.com)
snake in the grass (noun) 1. a treacherous person, especially one who feigns friendship.
Although many possibilities exist, I prefer using "devious" for this type of behaviour.
: willing to lie and trick people in order to get what is wanted
: not straight or direct
: having many twists and turns
Defined at Oxford Dictionaries Online:
adjective having or showing a cunning and deceitful nature.
"she had a sly personality" ... "sly as a fox"
This definition contains both the word cunning you were looking for and deceitful implying it fools others. So I think it would be a fitting choice.
You may refer to a person who practices sandbagging as a sandbagger. As indicated by the emphasized portion of the definition below, it speaks specifically to acting innocent.
A person who pads a handicap or acts as if he/she is at a lower skill level than he/she actually is so he/she can achieve better during competition that's handicapped or by skill level.
The Urban Dictionary
One of the verb definitions of sandbag is:
: to hide your true abilities or purpose in order to deceive people, gain an advantage, etc.
Sandbagging is a poker tactic, and is also known as slow playing or trapping. But, as seen in the definitions above, it can be used beyond the context of poker.
Slow playing (also called sandbagging or trapping) is deceptive play in poker that is roughly the opposite of bluffing: betting weakly or passively with a strong holding rather than betting aggressively with a weak one.
I like to think the term derived from a sandbag looking like a pillow.
But, alas, that is not reality. The term came from ruffians who literally took bags of sand and hit people with them. When the term was applied to poker, it was meant to denigrate a slow player, but instead it came to be a synonym for the tactic itself.Etymonline
A fox. As in the way the Lord Jesus used the term in the gospel of Luke, chapter 13 verses 31 to 33, about 2000 years ago.
The same day there came certain of the Pharisees, saying unto him, Get thee out, and depart hence: for Herod will kill thee. And he said unto them, Go ye, and tell that fox, Behold, I cast out devils, and I do cures to day and to morrow, and the third day I shall be perfected. Nevertheless I must walk to day, and to morrow, and the day following: for it cannot be that a prophet perish out of Jerusalem.