What is the ruling regarding a Muslim soldier who kills his Muslim brother while being coerced to do so?

Thereafter: Deliberately killing a Muslim is forbidden and necessitates entry into Hellfire. It is not permissible to proceed with taking a life that Allah has forbidden, under the pretext of coercion. Whoever does so bears the sin of killing and is liable for retaliation (qisas). If the killer knows that the killing is unjust, then both he and the Imam are liable for retaliation. However, if he does not know, then the sin and retaliation fall upon the Imam or the leader, unless he has a valid interpretation (ta’wil). The statement that "the killer and the killed are in Hellfire" applies to one who has no valid interpretation for his actions. As for one who has a valid interpretation, he is not held liable in his warfare for what was destroyed by the strife. And Allah knows best.