Just like Stroustrup's formulation, this can become a cover for unnecessary and mistaken excesses, but I don't necessarily think that's inevitable.
Every other Communist state that I am aware of also killed millions in internal purges: Cambodia and N. Korea, notably. I'm actually not sure what happened with Vietnam and Cuba. I'm not sure if contemporary Venezuela counts as Communist, but I am under the impression that there was killing or at least persecution of internal political enemies. I don't see how US sanctions have anything to do with how one treats political enemies.
I guess Eastern Europe might be an exception, but I think that is because Communist states were imposed with external force, not revolution from within, and the population mostly capitulated. However, I believe that political opposition was still likely to be deadly.
Since Communist states seem to be highly correlated with killing internal enemies, it seems like a feature of the system, not a response to external pressure, particularly since the largest two did not have serious external pressure at the time.