If the soviets had been true to karl marx’s theory of communism, what would they have done? why did hardline communist leaders resent mikhail gorbachev?

The Soviets were proclaiming themselves as communist, though the reality was not as the communism truly says a state should be run. If the Soviets practiced the textbook communism, then everyone in the society would have been equal. Everyone was going to earn the same amount of money and get the same things necessary for life without any problem. The economy would have been self-sustaining and that was going to be supported by excellent planning. There were not going to be social classes, but everyone would have been in the same class. The Soviets did pretty much everything wrong with those ideas, and in all fairness the human nature just doesn't allow for something like that to exist.

Mikhail Gorbachev was hated by the hardline communist. The main reason for that was that he was trying to make the country slightly more liberal, more open to the world and to modernization. That was seen as treason of the communist ideology by the hardline communist so they were against all of that.

eliminate individuals who opposed his rule

the enslaved people onboard suffered from illness with some frequency