Not a troll actually.
We did discuss the characters background story, and i have played with them several times before. I'd played dnd with them on and off for along time, and we have done an evil run before. It was the first time I had been DM, two of the players wanted to do some colorful stuff, and what they asked for their characters was okay. Those two were where the ones who skinned people and sacrificed people. The dragon born was a Barbarian who wanted to be a little paranoid, she had a pretty basic story about just wanting to get stronger. The most boring of the player stories.
We have always done several things, don't put alignment on character sheets if you want to hide them from other players, not the DM. The story itself was more neutral that players would be allowed to go in the direction they wished.
After the players started slaughtering the crew, two of them kept asking random crap, can I do this, can I do that, I take his severed head and put him on my hand as a puppet. I told them to stop, even evil charcters wouldn't do most of this stuff. It was mainly one guy that started it, and the others just went along with it.
Obviously i'm not giving every single detail, just a general desciption