There is no reason assuming you support the death penalty to oppose use of the guillotine. It's one of most painless methods of execution, and is one of the cheapest. Certainly much cheaper than lethal injection. So why isn't it in such use? Because it LOOKS gruesome. But if that bothers you, how can you support executions in the first place? They're OK as long as they don't LOOK bad?
People who support the death penalty but not the guillotine are hypocrites.
I support executions in Town Square, if there is no "Town Square" to hold the executions, build one!