France discontinued the use of the guillotine in 1977. I wouldn't say the French were barbaric and evil until then. It's just another method of execution. Traditionally, it's been one of the least painful when done right, with a guillotine or a headsman's axe, because death is instantaneous. What's barbaric about ISIS isn't how they kill enemy combatants - it's what they do to non-Muslim non-combatants, as well as Muslim civilians who disagree with their ideology.
I don’t think France is a good example of anything except maybe skin and fragrance products. Decorating maybe.
Regardless, the guy didn’t have a guillotine, he was not a skilled executioner, and he chose that type of death specifically for revenge.
You lose the high ground on moral issues when you start participating in the evil. We did not take German soldiers captive and give them a taste of their own death camp medicine. They deserved it but we didn’t. After the war there were trials and some were sentenced to death. It was not revenge. It was justice.