In my opinion, the main point of this essay is how immense of a thing it is to kill a human being. You can see this in a couple of ways in the essay.
First, you see it in Orwell's discussion of that very idea: he talks about how strange and wrong it is that the guy could be walking along, alive, and two minutes later he'll be dead.
It is curious, but till that moment I had never realized what it means to destroy a healthy, conscious man. When I saw the prisoner step aside to avoid the puddle, I saw the mystery, the unspeakable wrongness, of cutting a life short when it is in full tide.
Then, the idea comes up again after the hanging is over. The people who participated in it are acting strangely. Orwell himself feels the need to laugh. It's like they are a bit hysterical or on an adrenaline high because they know how terrible (I don't mean immoral here--I mean something more like "inspiring terror") a thing they have done.