What is the legal ruling regarding a commander who abandoned his soldiers in captivity and aided in their killing with the intent to undermine the army, and they were killed, yet he did nothing, and he knows their enemy and killer?

It is not permissible for a leader to abandon his soldiers or to aid in their killing. It is obligatory to support the oppressed Muslim and to repel anyone who wishes to humiliate him. An example of this is what Mu'awiyah, may Allah be pleased with him, did when a Muslim was captured and slapped by a Byzantine general. Mu'awiyah devised a strategy to recover both the captive and the general, teaching the latter a lesson, whereupon the captive exacted retribution from the general. This demonstrates Mu'awiyah's shrewdness and his concern for the rights of his subjects.