Beat Undertale Yellow pacifist, then neutral. The ending to pacifist felt really unsatisfying.
On my second time around in Shades of Justice, I thought that killing Guardener would have been the thing to tip me over into the Neutral, but apparently Ceroba didn't give a shit until I ended Axis.
Frankly, the whole ordeal with Axis didn't make much sense to me, but I didn't feel bad about killing him at all.
At least the neutral fight was really fun.