Plenty of bad leaders have their sympathizers, both civilian, government officials, police, and military. There is absolutely no one who can take over a country and control people all on their own. I just listed an example where in a public and filmed forum, a person asked a relatively prominent Republican figure when it was going to be times to start using guns. If Donald Trump (or whoever) started asking people to take up arms do you not think he would have at least a significant percentage of his supporters who would do it? Especially if they blame the other side for how their lives have turned out? Of course they would. Propaganda is a powerful thing and this country is filled with propaganda.
ETA: Nazi Germany started killing and raiding Jewish homes before they banned guns, but seeing as Jews in Germany at that time were a small population, there was nothing they could do about it even if they had guns. This whole idea of an armed populace being able to stop an authoritarian government is a complete hypothetical. Do you really think Jews could have defeated the Nazi military if they had guns? Of course they couldnt have. The biggest thing that leads to authoritarian takeover is the propaganda and ability to convince parts of your population, that another part of your population is your enemy. I think when we look back at Hitler and people like him, we forget that at the time, he had the support of many civilians who were either complacent, or even agreed with him about the Jews.
Another quick example. When John Brown started trying to free slaves, he was defeated by the US military pretty quickly and easily. And it isnt really debatable that the US at the time was authoritarian when it came to black people and slavery, and a well armed group of civilians was defeated quite easily. He wasnt trying to take down the government per se, but he was trying to fight against certain injustices, and the government beat him easily.