Well, for me it's usually in relation to my demonic nature. The urge to hurt people and take what I want from them. I can end up more overconfident, arrogant... Very possessive of things and people I might consider "mine", if that makes sense.

[He doesn't like it. Would prefer to not have to deal with it, but that's not really going to happen anymore, is it? So long as he has this mark, and is in this place, Kazuya will be subject and beholden to it's whims.

So the best he can do is try not to let it darken too much.]

Other suits have different effects. Hearts seems to make people more emotional, diamonds seem to act on more physical desires. Clubs is mental, things like that.