Well, there's three possibilities still, aren't there?

One: What you said. Whatever-duck killed Spikes with their nefarious evil knife plan or whatever and then ran into somebody else.

Two! Whatever-duck and Spikes had a tussle, and killed each other.

Three, Spikes killed Whatever-duck first, and then he tried to take out Alice-chan -- or the Alices -- by taking himself out. Didn't he say something about life-ties back when we had that powers thing going?

Pretty sure that's what we've got to go on right now.