What is the ruling on killing large lizards such as monitor lizards?
Question

What is the ruling on killing large lizards such as monitor lizards? Is there reward for killing them?
Praise be to Allah.Firstly:
Those lizards or other animals that are proven to be harmful should be killed, so as to ward off their harm.
But if it is proven that they are not harmful, then they should not be killed, like all other animals that are harmless.
The evidence for that is the proven report that was narrated in as-Sahihayn and elsewhere, according to which the Prophet (blessings and peace of Allah be upon him) forbade killing dogs, but he enjoined killing aggressive dogs.
Animals that are proven to be harmful come under the same ruling as aggressive dogs, which means that it is prescribed to kill them. Animals that are not harmful come under the same ruling as other dogs, which it is forbidden to kill.
Ibn Muflih said:
It says in al-Mughni, regarding the issue of killing dogs: As for those which are not harmful, it is not permissible to kill them. He quoted as evidence the prohibition on killing dogs. The wording indicates that other animals are to be dealt with in the same way. If it were permissible to kill harmless animals other than dogs, then it would be permissible to kill dogs.
This is what may be understood from the statements of the scholars, and it is a sound statement.
Based on that, we may understand that allowing the killing of aggressive dogs in particular means that it is not permissible to kill those that are not harmful."(Al-Adab ash-Shar‘iyyah 3/355).
See also the answer to question no. 159518 .
What is known about monitor lizards is that they are not harmful to humans by nature, because they are like lizards (ad-dabb). For that reason, it was narrated from some of the scholars that it is permissible to eat them. See Musannaf ‘Abd ar-Razzaq (4/529).
But some of the scholars said that it is haram to eat them, because they are regarded as vermin.
Ad-Dumayri said: Based on this principle, it may be understood that eating large lizards is haram, because they are vermin, and the scholars did not make any exceptions for them."(Hayat al-Haywaan  2/542).
Based on that, so long as monitor lizards are not harmful to humans, they are not to be killed, because killing them is destroying a soul for no reason, and undoubtedly it was not created except for some wise reason intended by Allah, may He be glorified and exalted.
One of the most obvious reasons is maintaining environmental balance, so that some animals will not overwhelm others. Monitor lizards are known for killing snakes, so killing lizards and eradicating them from the environment will lead to snakes increasing in number and becoming dominant. Glory be to the Most Wise Creator.
See also the answer to question no. 258988 .
Secondly:
If you find some harmful types of large lizards, or some individual lizards whose peaceful nature has changed and they have gone rogue, which is something that happens with many animals, and that results in harm to people, or there is fear of its harm, then such a lizard may be killed. The same applies if [a rogue animal] is attacking someone’s sheep or domesticated animals, and the like. He may ward it off by killing it in order to protect his property, just as any aggressive animal may be killed, even if it is an animal that it is prohibited to kill in principle.
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said:
Animals fall into two categories:
1.. Those which are harmful and aggressive by nature. It is permissible to kill them, whether they are those which the Prophet (blessings and peace of Allah be upon him) mentioned by name – such as scorpions, rats and aggressive dogs – or they are other animals which come under the same ruling for the same reason, namely that they are harmful. Hence the scholars said: It is permissible to kill every harmful animal. Such an animal is to be killed if it is usually harmful and aggressive, even if it has not caused any harm, because it will inevitably cause harm at some other time.
2.. Animals that do not cause any nuisance or harm. Such an animal is not to be killed, but killing it is not haram, although it is better not to kill it.
If it harms you, you may kill it in order to ward off its harm.
We only said that it is better not to kill it if it does not harm you, because with regard to what the religious texts say about animals and insects, they fall into three categories:
1.. Those which we are instructed to kill; 2. Those which we are forbidden to kill; 3. Those concerning which nothing was said about killing them.
Those which we are instructed to kill include scorpions, rats, aggressive dogs, snakes, geckos and so on.
Those which we are forbidden to kill include ants, bees and hoopoes.
Those concerning which nothing was said about killing them include all other animals and insects. Nothing was said concerning them, and it is better not to kill them, because the least that may be said about killing them is that it is destroying a soul for no reason.
Moreover, some of the scholars said that so long as the creature is still alive, it is glorifying Allah, may He be glorified and exalted, and if it dies, that tasbih (glorification of Allah) will cease, and your killing it means that it will be destroyed and will no longer be glorifying Allah.
Whatever the case, the ruling is that it is better not to kill it so long as it is not harming you, but if it harms you, then there is nothing wrong with killing it."(Liqa’at al-Bab al-Maftuh, no. 67).
It says in al-Adab ash-Shar‘iyyah by Ibn Muflih (3/355):
It says in al-Mustaw‘ib regarding things that are prohibited in ihram: With regard to ants and everything that is neither harmful nor beneficial, such as beetles, scarabs, worms, flies and ants that do not bite or sting, Ahmad (may Allah have mercy on him) said: If they harm him, he may kill them, but it is disliked (makruh) to kill them if they are not harmful, although if he does that there are no consequences for him. End quote.
See also the answer to question no. 127221 .
And Allah knows best.