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.

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.

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.

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

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

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

Go back in time and do something to prevent Vasily Aleksandrovich Arkhipov from becoming the Executive Officer on the B-59 Soviet nuclear sub in October 1962. He’s the guy who talked the Captain and the Political Officer out of launching the nukes when they thought they were being attacked by the US Navy during the Cuban Missile Crisis. His persuasiveness is generally considered to have avoided WWIII from starting then.

Let the Cuban missile crisis actually happen?

Procrastinate, the world is going down the drain just fine without my help.

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).

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

How much power do I have?

If I could divert the asteroid that resulted in the K-T event, that’d drastically change history. It may not have stopped dinosaurs from eventually going extinct, but it’d have given them 33 million more years more to evolve, and would certainly have affected mammalian evolutionary history. Maybe, just maybe, raptors would have gotten smart enough.

Ooh! Take back a lot of ravens. They’re almost smart enough already. Heck, I wonder if taking ravens back even earlier would be enough for them to evolve into something dominant. Problem is, they’re not particularly social, and I think that’s been our greatest advantage.

Or: introduce modern octopus to ancient oceans.

Stopping the K-T event is my favorite, though. It would absolutely have changed how life on Earth has evolved since.

30M years between extinction events is about all you get, though.

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

We’d still be living in medieval times.

You’re going to pretend we’re not on that timeline right now? I see your play, time traveller.