The reason they are perceived as foul is to keep us away.
Contact with feces or eating rotting animals is a good way to become sick and even die. So our ancestor that experience the smell as unpressed was more likely to stay away from it and more likely to survive. So we are the decedents of the ones that survived. So there is a selection pressure to not like the smell
In rotting flesh, it is the primary two compounds putrescine and cadaverine that is responsible for the smell. In feces it is skatole. The name is clearly from their this origin.
There might not really be anything special about them other than they are in/produced by the process and was a point that our ancestors start to dislike the smell of. It does not matter exactly what it was we started to dislike just that it is present in the air around it. It might be a chemical reason that makes the simple to detect or just that the early ones are released as a gas we can detect. But that has nothing to do with the experience of detecting them.
The experience of smelling them is subject to humans. Flys might be attracted to the exact same smell because they can lay eggs in it that can grow. Noting smells intrinsically good or bad it is only the interpretation of out brain.
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