Is the fly considered harmless by nature, and what is the criterion for harm that permits killing it? Does it differ from the harm caused by mosquitoes, which permits killing them? Is it permissible to kill insects whose harm is unknown? What is the ruling on killing ants when necessary, such as for cleaning?

If flies cause harm, there is no objection to killing them, and if it is proven that they transmit diseases, then they are considered harmful. Scholars differed on the ruling regarding killing insects; some permitted it, while others disliked it. You may adopt the view of permissibility. As for ants, killing them is forbidden unless they are harmful.

Killing insects is not commanded absolutely, nor is it forbidden absolutely. The Lawgiver has recommended killing some of them and prohibited killing others.

It is permissible to kill: snakes, geckos, and anything harmful such as scorpions, fleas, wasps, and bed bugs.

The Lawgiver disliked killing: frogs, ants, bees, hoopoes, and shrikes.

Jurists have made an exception for the permissibility of killing ants if they are harmful. The Malikis permitted killing ants under two conditions: that they cause harm, and that it is not possible to leave them alone.

The Hanafis and Malikis permitted killing insects, with the Malikis stipulating that the intention behind killing must be to repel harm, not out of mischief.

The Shafi'is divided insects into three categories: 1. Naturally harmful (killing them is recommended, like the five noxious creatures). 2. Those that are beneficial and harmful (killing them is neither sunnah nor disliked). 3. Those that have no benefit or harm (killing them is disliked, like beetles).

According to them, it is forbidden to kill Solomon's ants, bees, and frogs. It is permissible to kill small ants (adh-dharr) without burning, and also by burning if that is the only way to repel them.

The Hanbalis recommended killing any insect whose nature is harmful, even if it has not yet caused harm, and disliked killing ants unless they cause severe harm, as well as lice.

If you find that some insects are causing you harm, you are not prevented from killing them. As for those that are not harmful, there is a difference of opinion, and you may follow whomever you wish among the scholars. However, as for what has been forbidden to kill, it should not be killed unless its harmfulness is established.