Praise be to Allah, and may peace and blessings be upon the Messenger of Allah. To proceed:
A Muslim believes that Allah, the Exalted, is All-Wise and All-Knowing, and that He does not create anything without purpose. All of His actions are based on wisdom, even if some of that wisdom may be hidden from human beings.
Among the wisdoms behind the creation of harmful animals is demonstrating the perfection of Allah's power, the perfection of His creation, and His management of His creatures, despite their great variety and differences, and His provision for all of them.
They may also be a means of testing His servants. Whoever is harmed by them and remains patient may be rewarded, and through them the courage of those who protect themselves from their harm may become apparent. A person's faith and certainty may also be tested through them. The believer submits to Allah's wisdom and does not object merely because the wisdom behind something is hidden from him.
Another wisdom is to demonstrate the weakness and inability of human beings. A person may suffer pain or illness because of a small creature that is far weaker than he is. The existence of harmful creatures also demonstrates the blessing of beneficial creatures, for things become clearer through their opposites.
There may also be hidden benefits in things whose apparent nature is harmful. Some animal venoms have been utilized in medical and therapeutic fields. Many of these animals also serve as food for other creatures, thereby forming part of the balance of the environment.
Therefore, there is no absolute evil in every respect in Allah's creation. Rather, something may contain harm from one perspective while containing wisdom and benefits from other perspectives, even if a person does not perceive all of them.
This includes the creation of Iblis, for there are wisdoms in his existence, including testing the servants of Allah and distinguishing those who obey and strive against their desires from those who commit sins and neglect their duties.
Therefore, it is incumbent upon a Muslim to have certainty in the wisdom of Allah, the Exalted, and not object to His creation merely because he does not comprehend all of its wisdom. Rather, he should say:
“Glory be to Allah! How عظimate are His wisdom and His perfect management!”
And Allah knows best.