It's probably depending on how you were raised. You cannot really say it is a bad thing to kill animals, as long as you eat meat. Most farmers today kill a chicken for a meal, and when they used to it, they are not sentimental. I could not do it ( I don't even kill mosquitos), but I understand that that is somewhat hypocritical of me.
What I saw, was little Hassan looking somewhat anxious and fascinated, and his Dad talking comforting, maybe explaining to him. Maybe he told him that it is so quick that the animal doesn't really know, or something.
I think it is quite good that he sees that in a young age, while an adult can explain, that way it will become normal to him, as it has to be, as he will grow up with traditions like that.