The reason human hair makes a bad smell when burnt is due to the protein called Keratin that hair is made up of.
Human hair is made up of 90 percent of Keratin which is a protein and when burnt that Keratin emits an awful smell to it.
The Keratin protein that human hair is made up of is a fibrous structural protein and also contains sulfur which leads to the foul smell of human hair when burnt.
Human flesh burning is even more awful and I once witnessed a human body being burnt and smelled it during a house fire and it was very awful and that smell will never leave me.
The human hair burning is bad enough but when human flesh burns it's even worse.