Undyne the undying is the battle from the murder run that haunts me the most, but it was this moment that actually made me cry while I was doing it. It just totally devastated me. One of the earliest Undertale comics I ever drew was of this same moment, actually. This is a little inaccurate since when I originally got to this point, I did spare Sans, but the truncated version of his speech would not have the same dramatic impact, so in this one I chose to fight immediately, haha.

Shadow!Zar shouldn't usually be this clear but the body language/expression gets kind of lost otherwise.

Sans, the human (c) Undertale: the guilt haunts me still!