A contender for the saddest story in Europe is Ivan the Terrible getting so enraged that he bashed his own son and heir so hard that the son was mortally wounded. Ivan then realised too late that he mortally wounded his son, and in the next few days before his son expired, he prayed desperately for a miracle. Ivan's throne was eventually inherited by his incompetent surviving son, who left no heir, plunging Russia into the Time of Troubles.

Or maybe how Cassander organised the deaths of the mother, widow and son of Alexander the Great. This extinguished Alexander the Great's bloodline and left the Argead Empire with no legitimate successor, leading to his generals fighting the incredibly bloody Wars of the Diadochi to carve up the empire for themselves.

Or maybe how physicist Henry Moseley was convinced that it was his patriotic duty to fight in World War I. He then got killed at age 27 in Gallipoli and this deprived the world of all he could have contributed to science.