Moving the head at all and setting it up so doesn't, but that's what occurred. You'll have to forgive me for stretching the explanations here, but it is... Well.

[ He pauses, then makes a difficult noise. ]

Consider this: perhaps our killer was only trying to kill Alisha. Her death is one that's far simpler, and this is why I also believe that it's one culprit. This person wasn't trying to be malicious to start, but when they were interrupted, necessity said that they had to kill someone they hadn't expected to, and they also had to do it very quickly. Brutally, even. I don't think Gilbert was ever an intended target here, but because they were interrupted, he had to become one.

So the head itself could mean that perhaps it was like a distraction, a red herring, if you will? With our suspects, I don't know if we'll truly understand why they did it. It doesn't fit with any of these people as we know them, and yet, it occurred. The only people I would guess would be you and Undyne, but this is because you're both warriors. The idea of beheading someone wouldn't be naturally off-putting to you. But for the rest, it would mean we've misjudged them, and in that, the "why" doesn't matter, since we wouldn't be able to guess, would we?