Playing evil can be achieved by going down the slippery slope of morals.
You start with trying to protect a certain goal or value. And then you need to make sacrifices to achieve that goal.
A protagonist who has certain flaws can become evil given the right or wrong choices.
Some problematic person needs to disappear. To protect village X you need to get rid of village Y.
White turns into different shades of grey and eventually into black. It is a tragedy really.
I like how Pathfinder: WotC put forth the Lich path for example. My morally flawed sorceror did and sacrifieced everything to achieve his goal and became undeath.
