Why did the soviets kill ukrainian citizens

Because Ukrainian independence movement.

Explanation:

If the Ukrainian independence movement was successful will resulted in an economic blow for the soviet union since Ukraine is an important route that the soviet union has to pass in order to transport their oil product to western Europe.

So. the soviet created several plans to attack the Ukrainian citizens with the hope that the independence movement withered away with it. The most famous effort was probably the Holodomor. Holodomor was a man-made famine that planned by the soviet union to starve large portion of Ukranian citizens to death. They do it by eliminating major grain-producing areas of the country and prevent the citizens from obtaining enough food.