It would probably be WW2 and the millions of killings which occurred for no reason.
There are many events I would change however I think this was the worst disaster and affected so many that I would chose this one first to change.
I would change the results of the latest US election. More precisely, I would change it so Bernie Sanders became the Democrat nominee instead of Hillary Clinton. I believe he would have won the election.
The world is in very big trouble in terms of climate change and I believe Bernie Sanders would have helped the US in huge way in respect to the countries impact on the environment.
The crucifixion of Jesus and the Gallipoli disaster. The order to retreat came too late in Gallipoli and hundreds of thousands of men were slaughtered needlessly as they landed on the beach. The ANZACS were hopelessly outnumbered so were sent to their deaths due to the incompetence of the top brass.
I really cannot choose just ONE, and that is all there is to it. I'd have to say the 2 World Wars and ALL the horrors which not only happened DURING them,but the truly unbelievable manifestations which have damaged and hurt so very many people ever since! So much damage in so many ways...... So much sadness and irretrievable loss.... Such absolute cruelty and inhumane treatment of our fellow human beings. These wars changed Life as we knew it,and many still are suffering because of them.
WWII - my father was an officer in the RAF and was killed during the war. From a purely personal point of view it meant I would have grown up with a father. In a wider sense, still WWII because of the Holocaust which must be one of the most inhumane acts ever perpetrated on a race of people.