What would happen if Superman existed and he went bad and decided he wanted to rule Earth?
Hm. Now I wish I had read "Favored Son" which is "What happens if Superman came to Earth in Communist Russia....
1) If you are Kal-El, what is your strategy for world domination?
In the modern day? The "kill all the world leaders, upload it to YouTube, barge into the UN to make a speech" approach is pretty good.
Except... well, it has a major flaw. You are Superman. You are good at kicking butt and chewing bubblegum. Administration on a national scale is not part of your skillset. You need to have human world leaders running things for you. So killing them all, while dramatic, is not a great move.
So you don't kill the leaders. You kill all the *media* figures. All the movie and TV stars. All the top reporters. Oprah and a few others get handled in very flamboyant style, just because. And tell the world leaders that they'll be next unless they give in to your demands. Oh, and take out all the nuclear weapons, just in case they want to try a scorched Earth approach.
Not that I understand what your demands are. You are Superman. What can these humans do for you? You are already more powerful than anything these humans can build. What is the *point* of domination? Lex Luthor can want to dominate because there are things that humans under his dominion can do that he alone cannot. But that's not so for Superman.
2) If you are a [fictional] first world leader, what is your plan to prevent this, assuming alliance with the Kryptonian is not an option?
By the 1970s, in-canon Superman could take a direct nuclear blast. Heck, he could fly through the center of stars. There is *nothing* on Earth that can harm him. No poison, no force we can generate. Nothing. Nada. Zip. Zero. Zilch. Goose egg. I don't think you can prevent his takeover, except by threatening to scorch the Earth so it is just another rock. Be ready to lose some cities to prove that you mean it. But, of course, he can stop that too.
By what I noted above - maybe it really doesn't matter. Really, for the bulk of humanity, does having Superman at the top of the food chain matter *at all*? Depending what he wants, maybe you just feed most of the profits of the top 1% of earners in the world, and just leave it at that - what else is e goign to ask of us? Maybe the vast majority of the planet goes on just like it does every day.
But, let's assume he's going to be a complete sadist, and torture people by the thousands and stuff, cause he's bad. Superman really wants to rule, you can't stop him. But, can you *depose* him?
Maybe. Here's my idea - Superman is *not* an administrator, and the world is big. It should be possible to do things he doesn't know about. So, assume you can put together a modest biological laboratory without him finding out about it.
Now, you collect some super-poop. Yes, you need Superman's waste products. Because those will contain the only potential weapon on Earth to use against Superman. Kryptonian gut flora - we are going to try to give Superman the Kryptonian equivalent of dysentery, which can be fatal.
The prospect of a Kryptonian with explosive diarrhea may be one of the most disgusting things I can imagine. A horrible way to destroy a city, but, hey....