Life will find a way. Going back can be used to create nuclear winter but even then, life will go on.

To completely screw over the world, I would go into the future and acquire the source code for a general AI.

Then the world can be locked down and robots can be built to abuse every human and sentinent being individually, for a very long time.

Go back to the Garden of Eden with a bucket, grab all the apples, then head back.

Can I appear anywhere? Pop into white house or Kremlin during the Cuban missile crisis and say: I’m from the future, you must attack, or… Then travel back to the future without finishing the sentence.

If I ever got sick from something like COVID, I’d just go back to medieval Europe and let it spread like wildfire. I guarantee this modern disease would absolutely wipe out a large majority of Europe and everywhere else it spreads in the world. And little to no effort besides waiting for if I ever get it (which I thankfully never have had and hope I never will).

Let the Cuban missile crisis actually happen?

Stop Hitler’s parents from getting together in the first place.

Give the ancient civilization ruled by the most evil ruler modern weaponry and explain how it works

Buy a cheap pistol, go back in time, and shoot Gutenberg.

We’d still be living in medieval times.

My name is Friedrich Trump (aka Frederick Trump) and I already did that and even had a nice hump.

Bring COVID and HIV to the USA in 1929.

Maybe Zika as well.

Go to back to the big bang. You presence alone will butterfly effect the earth out of existence.

Kill Lincoln earlier during the Civil War.

Bring knowledge of CFCs to a time when we’re able to make them but not able to detect the ozone problems they cause.

Shoot Arch Duke Ferdinand. Few people even now about it today, but he was subject to a horribly mangled assassination attempt that many scholars belive would have set off a major war in Europe had he died.