Base the design on a Xikar Xi1 guillotine cigar cutter. Placed horizontally on an "execution table," with a hole through it that the condemned's head is inserted into.
Ceramic blades won't stain or rust from the blood and can go many uses before needing replacing. Power the blades with nitrogen-charged pneumatic cylinders. Before the execution, the blades are pushed back, fully compressing the gas cylinders, then locked into place. Insert head and at the flick of a switch, the blades fire and the head is snipped off in a thousandth of a second, slicker than a Fuente Fuente Opus X. Works first tiome every time and maintenance/reuse costs are near zero.