If I wanted to make the world worse? Nuclear weapons would be pretty quick.

Buuut why would I want to make the world worse? I’d want the world to be nice for me and other people and creatures.

Or do you mean, even if I act with good intentions, what sort of catastrophic errors am I most likely to make?

If that’s the question, then fascists would probably throw tantrums in response to my aggressively progressive initiatives and I’d get assassinated.