Leonardo da Vinci watched someone get murdered here.

Or "executed," if you want to be technical. But, in this case, there's very little difference. The Pazzi family aligned with Pope Sixtus IV against the ever-powerful Medici clan. Their first plot against Florence's preeminent overlords was pretty typical wealthy-people-conspiring kind of fare. The plan was for the Pazzis to finance a bunch of land purchases for the Papacy on the cheap in return for lucrative mining rights. They went for the slow con, which was The Pazzis' second worst mistake. The Medicis quashed the mine plan, so everyone involved turned it up to 11. 

The new plan was straight up ripped out of The Godfather playbook: Kill the Medici brothers at mass. The Pazzis only got one of the Medicis, their worst mistake, and Florence rose up to defend their tyrannical overlords. Go figure. The last remaining Pazzi was hunted down and hung by the neck outside one of the Bargello's windows. Leonardo was on the scene doing what he did best -- sketching the grisly mess for us to enjoy today.

The Bargello, obviously, wasn't always a museum. It's one of the oldest buildings in Florence, being built in 1255 by a commission from the People's Guard. They were looking for a fortress to survive a siege in case the wealthy families of Florence wanted to take all the power in town for themselves (spoiler alert: that's exactly what happened). The structure served as the home for the Captain of the People, the Florentine version of an American president. The next tenant was Florence's chief of police and, after that, the Bargello was turned into a prison. It wasn't until the mid-19th century that the Bargello, finally, became the museum it is today, displaying the greatest hits of sculpture the local archives have to offer.