you need technically to beat the game on the neutral route first time anyway (though if you killed someone you need to reset from the beginning rather than to continue just before the end), the game got a lot of several neutral minor endings too. I can see if some people are bothered by the game marking you for having done a genocide run, but you need to literally be doing it on purpose by killing absolutely everything. Casually killing monsters is not enough.

Resetted right after defeating Sans, the game gives a really good job at making you feel terrible... even some of Sans dialogue. Although I admit I laughed when I got "dunked".

I had a true reset right away after my first pacifist run so I didn't get to see any of the deja vu moments of the game.

I like both the ghost fight theme, which remixes the "Cafe" in earthbound with singing samples from the same game, and the True lab songs which are basically one big trigger of memories for earthbound fans regarding giygas (the amalgates orginates from Toby Fox's earthbound hack!). The theme that plays after you fought the final boss and can walk around freely in the world before the credits is using Chrono Trigger samples.

While I'm at it, in Genocide the ! for battle encounters is replaced by a smiley face. It resembles this thing a ton. In true lab it also appears in some encounters, probably to spook those who have done genocide run before. It also appears as a circular blob with a smiley face at one point like in SMB2 in the true lab just before the Endogeny encounter, but it's not pixel to pixel accurate so it might be a coincidence after all.