The examples you give are not examples of mistakes or unexpected outcomes. Regarding killing of kulaks, it's not like they wanted to achieve prosperity and the chosen measure failed; they intended to eliminate kulaks as a social class with loss of prosperity as an acceptable price to pay to get that outcome, and mostly succeeded in their goal. Regarding Holodomor, again, there's no mistake, the starvation was intentionally caused, having these millions of deaths was part of the goal ("the Sovietization of Ukraine, the destruction of the Ukrainian national idea, and the neutering of any Ukrainian challenge to Soviet Unity."), not an accidental side effect of agricultural policy choice. These are prime examples of harsh large scale practical actions that were reasonably effective in achieving political goals through mass murder among other means.

Effectiveness is about achieving goals, it's quite possible to be very effective while working towards evil goals.