I have to distance the character from myself in some notable way, because I tend to get too empathic. Not that it can sometimes be too easy to accidentally become a psychopath who kills everyone without feeling anything. The realization had kind of been part of a game of Skyrim I was playing recently, where I use some realism mods and put some limitations that I will never rob tombs, and keep looting of humanoid corpses to a minimum along with stealing, to recontextualize how a lot of games I thought I played the hero I was doing some morally questionable things. That game mechanics can make me kind of be the bad guy in ways.
Doesn't mean I don't like to occasionally try things on bad guy side. I had set up a couple of D&D campaigns where I wanted people to kind of play as the sort of monsters that are the bad guys, and go after bad guy things. I do find myself fascinated by the role of bad guys that monsters may have, a while back I even went through the D&D lore, where along with my knowledge of economics, I tried to put into context what economics could mean for varied races, including the monsters. Because people have funny things in thinking of economics as one thing, but with a little knowledge I could suss out depth. A friend also started a D&D pirate game, where it seems in the group I was the only one who went in with an evil aligned character (who cares mostly about money), and yet I was one of the ones not arrested because I did not decide to openly attack the law while outnumbered.
I have sure played some renegade characters, such as mean female commander Shepard, but it can still feel a bit hard when it is so in the face of choosing bad/mean choices. So there are either games where you are kind of playing the bad guy anyway, but the game humanizes it, so you can feel vindicated to be bad. Or to play on the evil side on a more player choice sort of thing, I have to justify it. Like letting it all loose as the Joker who thinks the world is crazy, in Fallout 4. Or a Khajiit assassin in Skyrim, who is fiercely loyal to the empire. Playing as a necromancer with a specific look in life in Skyrim, or a devoted to the Daedra Dark Elf in Skyrim. Letting out some dark side can be fun, but for me I think there has to be more complexity to a character rather than just say I am a bad guy, so I am going out of my way to kill anyone and everyone. I have had it in the back of my mind to some time play a racist Nord, for a different perspective, but I know for me it will be tough to get it right.
Maybe I don't fully belong in the parameters of the topic, because I often think that people are better than to really just be evil for the hell of it. There has to be a reason for it, and the true evil is something has really gone too far down the rabbit hole that I think can really be done in a game.
But kind of related, it is something I have been quite enjoying in recently playing Red Dead Redemption 2, because despite the fact I have been trying really hard to get good karma in the game, the white bar being full is one goal I have set myself, the game is really good at tempting me. The game rewards looting the dead, but trying to do it total innocents is kind of told to me that it is sort of a bad thing to do. And staying too long around the dead can often land me in trouble for something I might have even been the good guy in, because how would a random onlooker not know that I saved some innocent person, so to not get penalized, I might have to stop the otherwise innocent witness, which likely will have me doing something wrong. The game has given me good explanations of why my character is an outlaw, having me think how I have to act as one for my own good, what I have to say to live with those decisions, and then face that I still did something bad.
It is fascinating being the bad guy, if I can have a compelling reason why I am being such. Letting loose can be one aspect, but to me I can't be a lazy writer for characterization of the decisions made by me the player. And then there is beating people as Peach in Smash.
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Through dream I travel, at lantern's call
To consume the flames of a kingdom's fall