On one hand, the genocide run is fun because you do get different flavor text and exclusive fights. But on the other hand I hate how the Protagonist behaves. You should be portrayed as just a guy trying to survive, and aiming to save the world by any means possible. Basically a Renegade.
However the Protagonist is portrayed as a stark raving lunatic, who wants to kill everything on site. He does things that crosses the line into flat out cruelty. It’s as if the game is spray painting “THIS IS THE BAD ENDING RUN AND YOURE A BAD PERSON” on plywood, and beating you over the head with it. Personally I would prefer the Mercy run, but at the same time I shouldn’t be treated as an evil person if I kill monsters. Especially considering a lot of monsters in this game do really horrible things.