Yes, it does seem that a remarkable amount of Stellaris players love to play as genocidal hellholes. Dunno why, I tend to prefer playing as explorers in a galaxy empty of other starstates.
Come on, haven't you ever felt the urge to make the God-Emperor proud? There's even a catchy tune you can exterminate the alien to!I think it's because they can make everything they see in fiction series come true that basically describe genocidal empires as good things .
Or that they believe that it is unrealistic that space exploration developed in the style of Star Trek.
There are also people who play xenophile and they do quite well. I like the combination of being xenophile + having a strong fleet because everyone wants to be my friend.