The most likely cause for the slug's death when salt is poured on it is:
The high amount of salt caused the slug's proteins to break down in a process called denaturation.
When salt is applied to a slug, it creates a hypertonic environment, leading to osmosis. Water is drawn out of the slug's body into the salt, causing dehydration. This process can also cause the slug's proteins to denature, disrupting cellular functions and ultimately leading to the slug's death.