Disclaimer: This answer contains sensitive material and material that may hurt or offend some people. If your feelings get hurt, please, drop a comment and I'll see what I can do. The intention here isn't to offend anyone in any way.
Since most attacks generally start in the U.S.A lets start by.... WRONG! Lets start with China!
It is really cheap to make Gutter Oil, and we can make it cheaper.
Decriminalize the production of that oil, and make it mandatory to be consumed.
According to wikipedia, this is very toxic!
To help there, buy their medicine companies and sell placebo instead!
Export the oil to Japan and Korea.
Now, send false information to both Koreas and make them fight eachother.
That would be a really bloody war!
But war is great!
You can buy the companies who make both the weapons and the medical equipment to treat them.
You can infect them with HIV and, once again, distribute placebo to them and let them die.
Your budget is bigger now.
To fire up the war a little more, you can fake terrorist attacks on Russia, U.S.A and multiple parts of Europe (Germany and U.K. would be good places, right?)
This means more money since people will have to buy your bullets, guns and medicines.
If timed properly, you can climb yourself to be the President of the United states of America.
Once on top, you can bride them and change the laws to allow more than 2 guns per week for each person to buy.
You start a campain about gun safety to kids and hand them 9mm.
Send subliminar messages saying that life is like a videogame: that you die and respawn.
Kids shooting their parents everywhere!
Worst yet when the minimum age to buy alcohol lowers to 7 years old.
Since we are in a killing mood, allow untreated water or partially untreated water to go to people's homes.
Since you are on top of the economy, you can say that this measurement is to save some money.
Make it a law to underload the treatment centrals.
Just watch the quality of life, in general, to go down.
Now, send every single person who's 13 and up to war.
Send them in small batches, so they all die.
Use the media to deceive the U.S.A. and tell them that Europe is fighting Korea and pretend you are defending them.
Butcher them all down!
Kids and everyone on war!
The survivors would have a huge suicide rate which would help a lot.
Now, the economy falls down. Block all the importations and export everything.
Hunger and some deseases will kill a lot of people on the way.
Freeze everybody's salaries.
Mutins and Looting everywhere, with rebels setting fire to everything.
From here on, I think the destruction is self-sustained.
Nuclear accidents, sattelites falling down, people dying everywhere, no clean water, no food and a complete lack of protection!
This is how I would do if I were to destroy the world.