This is a very serious delicate topic that would definitely get me banned so i will stop here.
But, there are women out there, a LOT as sad as it is, some whom i've met, that genuinely enjoy this sort of evilness.
Viola liking doflamingo wouldnt be so far from reality based on what i've seen.
When i was a kid, there was a woman whose husband got killed living in the same building as my family stayed.
She had a child daughter and lived with her husband, the man got killed by a man who stayed in prison for a few months and once he came back, the dead guy's wife started sleeping with him.
As if it wasn't repulsive by itself, she offered her daughter to the man in order to "keep him home" and even spanked the daughter when she didn't comply.
It only stopped when someone saw the wounds on her body, took her to the police station and the girl confessed to the officers what happened to her.
A few years later, she ended up taking her life during early/mid teens.
I know it sounds made up, but real life is WAY worse than anything fiction come up with.
Thats why i dont see viola falling in love with doflamingo as such a far fetched thing.