I mean she is a extremely powerful fable, so why did she work for the Crooked Man when she could just brake his neck and take his throne? Was it for money? Respect? Or there was something i missed?
In one of her book of fables it says that she does it for fun because she was bored and it's a lot of fun. She'd even risk her life.
Shits and giggles.
I don't think Mary cares about power, or money, she's just in it because she's a psychopath and The Crooked Man gives her enough targets to sate her (on-duty) bloodlust.
She said it herself. She LIKES her job.
She wants to be on, what she thinks, the winning side.
Pretty much, crooked mans the only thing preventing mary going on a killing spree. crooked mans gotta give mary a target eventually, or she will snap of boredom and proceed to get alot of "stuff" to "feed the family dogs"... and if mary worked for anyone else she would of easily got too bored and kill her boss. maybe even the crooked man is a bit scared of her.
She's a creepy girl, he's a creepy guy. What else is there to explain :P