Sorry. Try to find for sure what it was so it doesn't happen again. If it was daytime, it could have been feral cat, dog,or hawk. Nighttime could have been coyote, fox, or owl. Also could have been mink or weasel. WARNING!!! When a predator finds a good meal, it will be back for more.
I see you are in Pakistan. What wild and domestic animals roam there? Even small predators that are hungry can attack and kill a young cockerel. A feral cat can do so, yes.
If it was a Cat should I kill it. I just found his tail feathers.