Did they first get the support of a vast majority of the voters then start purging rivals? Or did the populace not realize it was happening until it was too late?
I'm getting the impression that specific human lives were less valuable than what they mostly would be in modern America (maybe related to having big families because you'd somewhat expect one or two of your children to die during childhood), but still it's shocking so many politicians were executed in those times and people just accepted that as normal life? Or just looking the other way so they could continue getting the benefits from the Nazi and communist party membership in the early days of the rising to power?
Obviously there was lots of intimidation, but that can't just happen over night right
So it depends on each specific event but a good example is the night of the long knives. In 1934 Hitler had many top ranking Nazi officials and other political opponents murdered. These included men within the SA, more colloquially known as the brown shirts, most notably Ernst Rohm. Hitler was able to get away with this by claiming that it was in the defence of all of Germany. He stated in his July 13th speech to the Reichstag:
The National Socialist State will wage a Hundred Years’ War, if necessary, to stamp out and destroy every last trace within its boundaries of this phenomenon which poisons and makes dupes of the Volk
As he claimed that these men were causing problems and planning to harm the German state he claimed it was in defence and in that moment he was the protector of Germany. In reality he broke the law and killed people who opposed him and even some who supported him (see Ernst Rohm). This sent a message of fear to all who opposed him as, if he would kill his own men, what would he do to those who opposed him? It also made many believe he was the strong leader Germany needed to lead them through troubled times.
In that situation, Hitler made himself seem heroic and gained support through it but it does, like I said, depend on the situation. Eventually the situation in Germany became one in which people were simply too scared to speak out. If they spoke there would be repercussions but in 1934 Hitler could have been stopped but his cunning and his luck allowed him to solidify his power in a way which meant others could be dealt with, with minimum backlash.