Sadly, it was probably just survival of the fittest. You said the other chicks were eventually bigger than Fluffy, and chickens tend to pick out the weaker/weakest one(s), and mess with them until they die. I'm not saying that's what happened, but possible. I have raised many chicks in an aquarium with a heat lamp (they would never have survived in the chicken yard with all the adult chickens, they would have been pecked to death), and they turned out great. It's surprising how the death of such a small creature---sometimes meaningless to others---can impact us so hugely. I am so sorry you had to go through something like that, but it just shows you that no life is insignificant. Good luck with your other chick.
Everything can be achieved through patience!
I'd rather have a problem horse than a problem man. The horse I can work with. The man---I cannot help!