Oh sure, to get a good cut, you need a heavy blade.
The point is... who's gonna lift it up?
You can see the heft a blade meant for a single head has in this video from guys who made one this summer: https://www.youtube.com/shorts/RPO7fm_ZtlY (it's a short no long video)
I know the appeal of a guillotine is that it's fairly simple, but if we're gonna scale it up for maximal decapitating efficiency, then I feel like we can afford to have a motor with some horsepower lift the blade. Maybe two to make keeping both sides of the blade even easier.