You have a time machine with a single round trip. You hate everyone. And you are lazy. What is the least you can do to completely screw over the world with your one trip through time?

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 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.

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

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

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

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

Go back to before life formed on earth, and shit in the ocean.

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.

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

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

We'd still be living in medieval times.

Let the Cuban missile crisis actually happen?

Go punt kick the first fish back into the ocean as it tries to waddle it's fat ass out of the water.

I know that's not how evolution works but I can dream... Life would likely be vastly different given such a long period of time with something like that changing either way.

I hate the 'hate' part (pardon the pun) of the question.

I'd rather go back in time and get Jesus and bring him back here so that he can go all 'temple money changers' on today's MAGA Christians asses, give them a proper yelling to.

_{This comment is licensed under CC BY-NC-SA 4.0}

I really like this question. So: rather than killing Hitler, what if, instead, you killed Stalin? Was it inevitable that a strongman dictator would have taken over, and ruined the potential of communism? I guess we have evidence that the answer is "yes," in the form of Mao, but weren't the Chinese communist party(s) greatly influenced by the Soviet model? What if Russia had, instead, developed a more democratic system of government - was it possible, and couldn't it have affected how China's developed? But, maybe it is always inevitable that dictators emerge from internal revolutions like this.

Here's another scenario: what if you stopped Oswald, and prevented Kennedy from being assassinated? He was popular, and likely to win a second term. What would 4 (~5) more years of Kennedy look like?

Bring COVID and HIV to the USA in 1929.

Maybe Zika as well.

The thing here is that I have zero desire to screw people over.

Wait until I'm about to die, then go back to when the first land-dwelling animals first started coming ashore. I'd bring a bunch of cockroaches with me and then I'd die there. Either the roaches or my decaying corpse will hopefully cause enough change to the timeline that humanity never develops in the first place.

I think that's the most I could screw over the (human) world.

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

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).

Go back to 2011ish and unleash WanaCry on a world that is not ready for for a global crypto locker.

I'd kill my parents before I was born.

I'd kill Harambe. Wait a second...

I read about a study that said Homo sapiens may have been down to as few as 40 breeding pairs at one point.

I’ve got way more ammo than that in my closet.

Get COVID, go back to the neander valley and cough.

Lowkey, just travelling back may be enough with the amount of diseases we would carry with us

Hmm I don't know. Probably go back to Germany 1940 and give them all the information needed to develop nuclear weapons first. You just know that Hitler would've pushed the red button a hundred times over if he had the chance.

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

I'm heading far into the future - say 1000 years or perhaps 100 if communication would be an issue. I'm going to grab all sorts of portable devices and tools to take back with me, and I will maintain the remainder of my life without sharing my goodies. The world is screwed because they get no fruits of my intense labor, but that is fine since I hate them anyway. For that afternoon of work, I get to live a great life while everyone else must suffer in modernity.

The biggest killer for indigenous people is said to be infections - bacteria and viruses. Collect some in this time - maybe a cold - and go back and presence will be enough.

It depends on how you interpret "screw over the world", but I guess "you hate everyone" implies humans. If you go back to the first humans from Africa, before spreading out, that may have the biggest impact - significant delay or suppression in development and spread, or eradication.

Go back to the stone age and drop modern steel tools and weapons where a tribe could find them. That tribe would likely become dominant and perhaps change everything.

"so there's this dude named God..."

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

I kill Dr Morell before he meets Hitler.

I could skip my next union meeting, I guess...

Kill all the chickens before humans had the chance to domesticate them

Tell Hitler to focus on Operation Sea Lion and to leave Russia alone

Kill Lincoln earlier during the Civil War.