I’m talking about events on a society-level scale, involving the beliefs and actions of a mass of people. The decision making that led to the event could be logical, but the consequences (whether contemplated beforehand or not) must be insane.
My vote goes to the Battle of Verdun. The Somme and Stalingrad were bloodier, but they were fought in pursuit of rational, logical military goals. WWI itself is rather insane in hindsight, but the motivations of the combatants in starting the conflict made sense in light of the history of Europe and the Great Power mindset that existed at the time. Verdun was simply nuts. Given the inability to fight a war of movement, the battle essentially amounted to an agreement between France and Germany to meet at a battlefield and bombard the crap out of each other for several months, with the hope that the other side would suffer more. Germany had no hope of forcing a decisive advance, and France wouldn’t think of surrendering the position, so they just stayed there and slaughtered each other. The rationale was basically “We will manufacture weapons and supplies, equip men with them, then send them off to Verdun to die. We will continue this for as long as necessary.” It reminds me of the tale of Shaka Zulu ordering one of his Impis to march off a cliff to showcase their discipline.
I would go so far as to claim that the Holocaust and the aerial bombardment of civilian populations during WW2 made more sense than Verdun.