Dunno, I did Ralsei Mode (you don't actually kill anyone, so it's ralsei and susie mode hehe). I think you have to bow? It doesn't really matter, the difference is mostly between the neutral and pacifist routes.

I looked it up and it took some searching but I found that in order to kill him you have to warn him about Susie 3 times, which makes her attack rise, then get your gits perfectly every time. Hits as in, the little box thing with the sliding bar that shows up when you determine(ation *high fives Sans*) your hit's power.