I never even considered one might play Undertale where you only kill some people but not others, the game doesn’t really ever incentivize you to do a “mixed” run.

Pacifist is more like a puzzle game, and I didn’t find it all that challenging at all.

If you go the “kill only some people” route, I don’t think you would ever really be strong enough for this to be fun due to not levelling up enough to keep up with later stage enemies. So you’re going to be heavily incentivized to go full pacifist to bypass levelling entirely or genocide to get enough levels. Then of course you’ll hit the wall at the end…