It is my understanding that executioners, at least in the Western world, were despised, detested and generally considered to be at the bottom of society. But why was this? Violence was, after all, perfectly acceptable, even lauded, in other circumstances (warfare, defence of ones honour). What made an executioner different from a soldier? An execution isn't a "fair fight", of course, but on the other hand those sentenced to death were criminals who had often committed heinous acts, and it doesn't seem like the mass of the people were opposed to the death penalty itself.
If anyone is able to shed some light on this to me somewhat puzzling question I would be most grateful.
I can not answer for all of western society, I can however answer for Sweden specifically. Executioners were, in terms of respectability, the lowest of public servants. Occasionally they were criminals who were granted a stay of execution in exchange for their service as executioners (it was sometimes the first duty of the new executioner to carry out the sentence against the old one) but for the most part they were promoted from among the helpers of the executioner.
So why were they such a disreputable bunch? Well. Cities were not the cleanest places to live but someone had to quite literally deal with the worst shit. This was the job of the executioners aids (called "Rackare" which roughly translates to rascals in Sweden), they transported human refuse out of the city at night along with dead animals and other... well basically really disgusting stuff.
This kind of job was rough. We can see from public records that they were frequently reprimanded for being drunk, getting in fights or engaging in lewd behaviour. The job was shit and the pay was bad, your boss was frequently a criminal convicted to death. This is not going to attract the best and brightest candidates. Consequently the people who did it for a living were looked down upon.
Oh and to add something about criminals serving as executioners, they were frequently mutilated to discourage any attempts at escape. Having your ears and nose cut off was not unheard of, neither was disfiguring brands.