Abhimanyu's death.

You can justify almost anything if you try hard enough. Even war. And deaths in war. Thousands of soldiers die in the line of duty every day. And somehow, we have come to terms with it and deluded and convinced ourselves somehow that it is fine. After all, as long as there are humans, there will be tiffs, disagreements, fights, and wars. And able-bodied, young men and women will die trying to protect their country, their beliefs, their families. 

But what cannot, under any circumstance, be justified is cold-blooded murder. Yes, that was what it actually was. It wasn't "part of the plan". Abhimanyu wasn't just another war casualty. He was murdered. By some of the best warriors and respected individuals of the age. In full possession of their mental faculties. In clear violation of the Kshatriya battle code of conduct. 

Fighting with such ferocity, none from the Kaurava side could beat him in a one-on-one combat. Proceeding, Abhimanyu defeats all the maharathis in a one-on-one battle, highlights including injuring Shalya so badly which causes him to faint, killing Shalya's younger brother, piercing Drona and Kripa, injuring Dusahshana badly, piercing Karna and cutting off his bow, killing Karna's younger brother and Brihadbala, the king of Kosala of the Ikshwaku dynasty.

Abhimanyu really proves very expensive for the Kaurava army, destroying 3/4th of an akshouhini army single-handedly. Incensed at the admiration that those like Kripacharya and Drona express for Abhimanyu, Duryodhana attacks the boy. Sparing him so that his uncle Bhima can fulfill his oath, Abhimanyu leaves Duryodhana chariotless and weaponless, bleeding all over. With a single arrow, he also kills Duryodana's son Lakshmana, who had come to defend his father. Enraged, Duryodhana orders all the Kaurava maharathis to attack Abhimanyu, who with disgust, counters all their attacks. He then attempts to arrest Duryodhana.

Following the orders of Duryodhana, the Kauravas attack Abhimanyu from all sides. Karna cuts off Abhimanyu's bow from behind . Then Drona and Kripa kill Abhimanyu's horses and charioteers. Abhimanyu draws a sword and shield and continues to wreak havoc on the Kaurava army, only to be disarmed from afar once again. Drona destroys the hilt of the sword and the shield. Abhimanyu, who famously uses the wheel of a broken chariot to fend off attackers in lieu of his weapons. The wheel is broken into fragments by the enemy forces. Abhimanyu picks up a mace and pounces at Ashwathama. Ashwathama leaps back to avoid Abhimanyu. A mace battle between Durmasana, the son of Dushasana and Abhimanyu ensues. During this mace battle, both of them are thrown to the ground. But Dushasana's son rises first and hits the crown of Abhimanyu's head as the latter was in the process of rising up from the ground but Abhimanyu killed him before he died. Karna kills him by stabbing for taking revenge becauseArjuna killed three of his sons. Thus he was killed by seven unequal veterans in the battle.

(Source: Abhimanyu)

So, 7 of the greatest warriors of their age had to come together, plot, scheme and abandon all morality, because a young, 16-year old boy threatened their battle prowess.

Whatever little salvation or empathy remained for the Kauravas, for Karna, for Drona, for Ashwathama, all of it vanished when they decided it was better to win like a coward than lose like a warrior.

Abhimanyu's death proved that life isn't fair. It proved that men and monsters reside together within. It proved once and for all that there is no ambiguity in the Mahabharata. That the Kauravas are pure evil. And no, Karna, you weren't the best warrior of your age. You were no saint for being generous with alms. You do not get any points for the wrongs done to you.
You were a cheat. Abhimanyu was a better warrior and a better person.
Source:Quora(Deepak Mehta).


(A rendition of the scene; courtesy: Abhimanyu - Last afterburner)