I actually support the guillotine because while certainly messy, it's the most humane method of execution ever invented as far as I'm aware. It has a decidedly ugly reputation, sure, but I largely consider that a feature rather than a drawback because it quite neatly satisfies the deterrent half of the equation. The actual execution is as humane as possible -- beheading with mechanical precision, no hesitation or inaccuracy, near instant total loss of blood pressure and volume to the brain.
I find lethal injection deeply immoral because it pretty much is tailor-made to look as humane as possible while actually being horrifyingly inhumane. I'm not a fan of conspiracy theories, but I find it difficult to believe anyone could possibly actually invent it without that being the exact intent, because there's almost infinitely many ways to poison someone, and the one they picked is just up there. You could painlessly execute someone by the exact same method that is used to put pets to sleep (massive overdose of tranquilizer), or you could use any of a vast number of fast-acting poisons to render them "dead before they know it". Instead, the method chosen is to use a limited dose of sedative combined with a paralytic agent to immobilize the subject, then slowly administer an electrolyte to shut down their heart. At best, it's an unnecessarily complex protocol that doesn't actually guarantee unconsciousness. At worst, it's a protocol that literally only makes sense if you wanted to make the subject suffer as much as possible while making it difficult to *prove* they were suffering.
The electric chair, meanwhile, *actually was* a conspiracy; Thomas Edison was heavily involved with lobbying to use alternating-current electricity for executions specifically to promote the idea that the alternating current systems produced by his competitors were unsafe compared to the direct current systems made by Edison. The Edison company provided all of the equipment to develop the first experimental electric chair for human executions, but illicitly obtained Westinghouse AC generators for it to make sure that the rival brand was specifically associated with electrical executions.