First I’d like to go back to the OP and point out that there are some real large differences between Idi Amin and Pol Pot on one hand, and someone like Hitler or Stalin on the other. Remember, Hitler came to power in a Democratic government, pretty much telling everyone what he was all about. He was a bit vague on the specifics of his Final Solution, but getting those treacherous Jews out of the Fatherland was one of the key points of his public policy. Likewise he was going to make Germany great again with the implication that it would be happening to the detriment of Germany’s neighbors. Stalin enjoyed bragging that his constitution for the Soviet Union was the only Democratic constitution Russia had ever had.
Also, Germany and the Soviet Union were not regiemes that could be weakened by simply refusing to allow an arms trade to exist with them: both were modern industrial nations which produced large amounts of war materiel completely from their own resources.
A lot of the question of how a pacifist will react to an unjust and repressive regieme is going to depend as much upon the specific nature of the regieme as upon the specific nature of the pacifist in question. As clairobscur points out, there are lot of different stripes of pacifist, ranging through a whole spectrum of reactions and beliefs.
In Germany, for example, there were a number of people, even during the war, who continued to speak out as best they could against the actions of Hitler’s government and the SS. IIRC most of the public opposition ended up being sent to extermination and work camps like the Jews and Gypsys. However, the surviving writings of the people in such groups makes it pretty clear that this was not something they didn’t expect. Opposition in the Soviet Union was handled almost the same way, and it was still there, both violent and pacifist.
During WWII there were all sorts of ways for Conscientous Objectors to contribute, based upon their own beliefs. There were industries that were protected against the draft, even then, and even growing food would keep the industrial might of the US strong enough to support the war effort. I’m not sure, in a modern economy, it is possible to claim that any economic action isn’t going to support a nation’s military efforts in the kind of war that WWII had been.
Then there are those Conscientous Objectors who wouldn’t carry arms but were still included in combat units. IIRC there’s at least one Medal of Honor holder from Guadalcanal who was awarded the Medal for his courage in treating and saving other men in his unit.
It’s harder to say what should be done about a unfair regieme like that of an Idi Amin or a Pol Pot. During the disintergration of Yugoslavia, arms sales to the Balkans had been banned. But, since the factories in Serbia were still making large numbers of war materiel, this ban had the effect of leaving the people most at risk unarmed and unable to effectively defend themselves. OTOH, the supplying of arms to anyone willing to fight the Soviet backed puppet government in Afghanistan has helped create the chaos that is still going on there.