"If they thought you really were a threat you were executed rather than being sent to a camp."

True, but the threat of deportation was nonetheless one of the Nomenklatura's weapon of choice. Someone's death was, even in the aftermath of the WW2 mass carnage, perceived as brutal mean of "solving" problems and could spur further backslash/dissidence. Someone's separation from their social circle however, was a much more safer method and the perpetrators were able to sleep at night because there could hardly be any retribution for such a thing.

There was a very specific intent of extermination in the slave labour thing, not just utilization of slave labour. Political prisoners would be killed by having too little food when doing heavy work, while ordinary criminals, who would have been equally useful as slave labour, were given a privileged position. [0]

While at this, Soviets also made experiments with people, and I think this was to some extent done out of sheer curiosity for information.

For instance, take a group of men, split them to three and send them to build a canal (dig earth with shovel, cut down trees with hand saw, use these methods to build embankments for a canal).

One of the groups gets "normal" rations (which are far below what is needed to survive). One group gets a little bit more bread, another group gets even less. Then set them to work, and observe how quickly each of them dies. This enables you to optimize a conversion ratio of bread-and-people to kilometres-of-canal.

Soviets studied this, and their then-friends the Nazis came to observe and study and then refined the methods later on.

[0] https://books.google.fi/books/about/Kremlin_kellot.html?id=K... (written by Arvo "Poika" Tuominen, a Finnish communist who was in close contact with Stalin in 1930's. Unfortunately only in Finnish)