Is it permissible for a citizen to kill those who commit hirabah (unlawful warfare/banditry) that is widespread, given the absence of security to deter it?

A person must repel an aggressor who attacks him, his family, his children, or other Muslims, even if it leads to killing the aggressor. This is based on the Prophet, peace and blessings be upon him, who said: "Help your brother, whether he is an oppressor or oppressed," and his saying: "A Muslim is the brother of a Muslim; he neither wrongs him nor abandons him." Furthermore, killing such an aggressor does not incur liability.

Repelling the aggressor should begin with the easiest means, then progressively move to more forceful ones. If the aggression cannot be repelled except by killing, then that is permissible. Similarly, if it is highly probable that the aggressor will immediately kill, then killing him from the outset is permissible, while still observing gradual escalation and ensuring that the defense is proportionate to the need.

Repelling aggression is obligatory when there is an attack on one's life, honor, or children. As for property, repelling aggression against it is permissible but not obligatory. If the person being attacked is killed, he is considered a martyr.