I'm really confused about how you have come to the conclusion that he must have killed it in an inhumane way? I keep poultry, ducks, hens etc and have had to cull due to illness many times. It is considered kinder to do this yourself, if your birds aren't used to lots of handling than taking a bird to a vet, because they are not remotely used to being inside human and domestic spaces and the fear they feel means they aren't getting a "kind" death when pts at the vets. A wild bird would have been incredibly stressed being taken to a vets as you suggest would have been kinder.
Magpies are very clever and a bird like this that was clearly very friendly and unafraid of people is likely to have been hand reared or had some kind of stay at a wildlife hospital in the past. Unfortunately humans intervening and hand rearimg birds without planning a site to release them to leads to birds like this one that can become pests.
We are in the middle of the worst avian flu outbreak in years in the UK. I am certain that with that in mind, if this manager had sought advice about a wild bird coming into a shop which sells food he would have been advised to have it dispatched. You can't trap and relocate birds like this as there is a risk they will become nuisances elsewhere. Rescues are not taking in birds unless in emergencies atm due to the avian flu outbreak.
In terms of dispatch method I would be very suprised if it wasn't humane. Magpies are small birds and it would be relatively simple to snap its neck which would kill it instantly. That or a sharp hit to the skull with a brick which again would have been a very quick death.
As has previously been pointed out, all of the above aside, you have no way of knowing whether the bird had been injured by a bicycle, car, person, cat or fox. Overly tame birds very often fall prey to cats and foxes especially in urban environments.
Animal welfare is less about a black and white "keep everything alive at all costs" and more about understanding the bigger picture. There are so many factors that you don't know and haven't taken the time to understand, and instead have jumped to the assumption that this was a poor healthy innocent bird, trampled to death by a psychotic lunatic.