Abhimanyu was killed in a fair war if we read the scenario of whole war. He had a lot of advantage in the war. Let's read about his advantages and later we I crush the misconception regarding the war.
Advantages of Abhimanyu
He was under the protection of a celestial armour protecting him from the attacks of warriors.
Abhimanyu is young, his prowess is ...
It is Arjuna who worries about varna shankara. Krishna ignores Arjuna's remark. Instead he talks of Varna being based on karma and guna.
According to the aptitudes resulting from the dispositions of Nature
(gunas) and works (karma), the social order of fourfold division has
been created by Me. Though I am their originator, know me to be an
agent but ...