Stellaris is actually the first game I've ever actually been able to revel in being the bad guy. I can't make myself do that in other games where that's a thing, such as RPGs with a morality system or otherwise branching paths where the player character has the option of being the villain or the hero or anything in between. I usually have something resembling a hero complex, but Stellaris is different somehow.
Oh, I started as a "good guy" nation, but as games went on and it became apparent to me how stupid the AI was, I quickly became a xenophobic control freak, at least when I wasn't playing an outright isolationist or genocidal nation.
That said, in the end, I always want to go back to being the good guy. After seeing certain posts in this very thread, I start to want to become very protective of certain varieties of empire whenever I can find them, particularly the xeno-compatibility ones. I believe it is the right of all sentient beings to be free, and that goes IRL as well. So if I catch someone bullying a xeno-compatibility empire, they may very well find themselves in a galaxy – nay, universe – of hurt.