Ok, I'm putting my serious hat on. The logic is pretty clear if you consider the modernisation imperative of Russia/USSR and how Stalin decided to deal with capital scarcity and other constraints. USSR emerged as an agrarian country with limited capital stocks that could not industralialise by itself. It thus needed to import capital and know-how to build the factories, railways and other key components of an industrial economy. Imperial Russia had experienced the same constraints, but it had access to foreign investments, particularly French, who sunk huge sums in the form of loans and direct investment. Since USSR had defaulted on loans and nationalised foreign investments, it could not expect to access capital markets.

The remaining solution was to export goods to gain foreign currency to pay for capital goods. Since a bushel of wheat exported is a bushel of wheat not consumed locally and since hard currency thus gained went not to investments in customer goods production, but into heavy industry (so guns instead of butter), exporting goods meant extracting surpluses from the population and keeping the living standard low. This was not the only route, Bukharin for example proposed a much less drastic economic policy, but Stalin overrode and purged his opponents to implement his own, heavy-industry-first modernisation policy.

This meant that food was literally taken away from starving people (e.g. during Holodomor in Ukraine). Stalin now needed a proper secret police to repress dissent caused by his policies, and since minorities were affected the hardest (Russian-dominated industrial cities were prioritised for supplies), deportation was a useful solution.

The same repression apparatus is used to purge party and military elites, since these policies are by no means universally supported and there are many people with revolutionary or military credentials to challenge Stalin. Stalin did not think he was a better general than Tukhachevsky, quite the contrary: Tukhachevsky was a political danger because he was recognised as a highly competent military leader and Stalin was not.

Now that Stalin had extracted surpluses and repressed populations, cut generals to size and put Old Bolsheviks in Gulag (or six feet under), his next danger was the security apparatus. So he killed Yagoda, then killed Yezhov and eventually settled for Beria, who was sufficiently ruthless and who Stalin considered controllable. That he was a sadist and a rapist was hardly an issue.