We should accept it was unjust that all of them killed Abhimanyu in the war. But if they did not kill him, he would have wrecked their army and would have brought in many allies; having no options, they killed him in an unrighteous manner. Even the Pandavas indulged in unfair means during the war.
To emerge victorious, we have seen many resorting to unscrupulous ways in the Mahabharata war.
As for Dronacharya and Krupachariy being Brahmins, it is an accepted law that when Brahmins were left with no options, they can take upon the ordains of a Kshatriya. We can pacify ourselves that to protect the country they participated in the war.