WMDs like Brahmāstra and Brahmaśira should never be fired on unarmed civilians or even enemy combatants who are no match for these weapons. Otherwise, it can set the whole universe (Earth?) on fire.
Droṇa warns Arjuna about the dangers of Brahmaśira before bestowing it on him:
Adi Parva (Sambhava Parva)
. . .
A few days later, the best of those of the Angirasa lineage went with his students to the Ganga to bathe. When Drona entered the water, a powerful crocodile grabbed him by the thigh, as if sent by destiny itself. Though quite capable of saving himself, Drona told his students, "Kill the crocodile and quickly save me." Even before he had finished speaking, Bibhatsu let loose five sharp arrows that killed the crocodile under the water. The others were still standing around, looking confused. On seeing the Pandava’s swiftness in action, Drona was extremely pleased and decided that he was the best of his students. The crocodile was chopped into many pieces through Partha's arrows. It let go of the great-souled one's thigh and returned to the five elements. Bharadvaja's son told the great-souled maharatha, "O mighty-armed one! Receive this invincible and supreme weapon, named brahmashira, with the knowledge of releasing it and withdrawing it. You must never use it against human beings. If it is used against an enemy whose energy is inferior, it will burn up the entire universe. O son! It is said that there is nothing superior to this weapon in the three worlds. Therefore, preserve it carefully and listen to my words. O brave one! If a superhuman enemy ever fights with you, use this weapon to kill him in battle." With joined hands, Bibhatsu promised that he would do as he had been asked and received the supreme weapon. His preceptor again told him, "No man in the world will be a greater archer than you."
[The Mahabharata: Volume 1, Bibek Debroy]