What happens if one eats "1 kg salt" at a time?

It would almost certainly kill you. If you got medical help REALLY quickly, they might be able to save you.

At least in rats, the median lethal dose is 3g per kg of body mass. Me, I weigh 100 kg. So 300g of salt would give me a 50-50 chance of kicking the bucket, assuming my body works roughly like a rat's. 1kg is more than 3x that.

This is a very famous Chinese torture/suicide technique, where a person was made to eat a lot of salt.

Salt would lead to water leaving the cells of the body, which would result in the death of these cells. Excessive intake of salt leads to hypernatremia(an excess of salt), the symptoms of which are neurological-unresponsiveness, lethargy, weakness. Too much salt, and it can cause seizures or a coma.

Crudely summarizing, the salt in the stomach and guts caused a severe imbalance of tonicity between the insides and outsides of cells, i.e. salt concentration was considerably higher on the outside of the cells lining the stomach etc.'s inner walls. What then happens is that water is rapidly pulled out of the cells, which disrupts pretty much all of the cell's (water-based) chemical operations. Cells die. This poor infant suffered massive tissue death, starting in the stomach and guts, I guess.

To the extent that parts of the body are still functioning, a lot of that salt will also have gone into the bloodstream, where the kidneys would make a valiant effort to pump it back out. I'd expect the kidneys to fail next, then.

Meanwhile, the operation of the heart, brain and pretty much everything else in the body, particularly the nerves, requires a sensible balance between the electrolytes sodium and potassium. But the metallic ion in table salt is just sodium, and that would have tilted the chemistry of various organs, lethally.

A person in India actually did this. He was a teacher in government school and ate 500 grams of salt in single sitting, you guess it just for the sake of 'bet' with his friend.

He died within an hour. His post-mortem report said immediate kidney failure followed by failure of other vital functions.

A local Hindi newspaper dedicated a small, four-line mention in the news for him, treating it like a minor incident.

This is the same reason it is recommended that sea wreck survivors should not drink a lot of sea water.

 This is the reason why many people drink concentrated salt water as a first aid measure in case of suspected food poisoning.

Lets assume that miraculously, a person manages to ingest 1KG of salt. This would lead to severe loose motions ( for the same reason mentioned above - in this case, the pressure is built up on the intestines ). The cells would start shrinking due to water loss, to a point wherein the normal cellular functions are disrupted due to absence of water. This is the point where the organs start failing and resulting in severe painful death.

My advice, do not try this at home and reduce the general intake of salt in daily life.