I personally would nominate Gavril Princep’s decision to go for a sandwich after failing to assassinate Frank Ferdinand. That led to him being present when the Archduke and his wife passed by and his assassination of them. That triggered the July Crisis, which led to the Great War. Princip himself died in prison before he could turn 25 and unleashed the horrors which would go on to engulf the world I er the course of the following century.
Granted there were a lot of ancillary aspects to the Great War and what followed, but this has to be one of the most disasterous decisions taken by anyone in its impact on him personally, on his nation (leading to the death of a large percentage of his own ethnic group and worsening the inter-Balkan relations considerably, setting the stage for all the Balkan horrors to follow) and on the world as a whole.