Abhimanyu entered the vyuha which he thought he knew to break. The details were embedded in his sub conscious not because he had learnt or studied it.
The reason why it became a brutal massacre was because his slayers flouted all rules. It was always to be a one on one attack but the learned Kauravas chose to ignore it
It has to be a combat between equals yet the maharathis and Adi Rathi's attacked a warrior or their equal in rank.
The weapon used should be similar yetthe multi pronged kaurava attack did not adhere to it.
Again we say that blood is thicker than all else. Arjuna was in amoral dilemma to fight his cousins, to fight his teacher, to fight his adversary in a battle of this nature.
The kauravas attacked Abhimanyu without qualms.
Drona is beholden to Arjuna who avenged Drona sinsult by drupada, Ashwathamma claims Drona loved Arjuna more than him , he he chose to attack Arjun's son without a second thought.
Karna I think knew by then he was a kauntheya...he knew he was Arjunasoldest brother yet he chose to kill the nephew.
Warm is never the means to settle any matter. Allowing weapons access is never justified. There is an article on the bombing of Hiroshima where the American author says it is an aberration for which America can never be forgiven.
The manner in which Abhimanyu was killed was inhuman and so it is held as an example of unfair battle.
A reluctant Achilles entered the trojan war after his young cousin was killed mistaken to be Achilles as he wore his coat of arms. Abhimanyu a case they knew they were right information him. Just as the entry of Achilles marked a turning point and killing of Hector the fall of Troy.
The intent of several wars is to proove a point and avenge an insult be it the abduction of a beautiful queen, disrobing a DIL, attacking Pearl Harbour ( US peeved by the act bombed Hiroshima and Nagasaki)