Praise be to Allah.
There are two aspects to the answer to this question: general and specific. The general answer is that the Muslim believes that Allaah is All-Wise and All-Knowing, and He does not create anything in vain; all of His actions are based on Wisdom. If the wisdom behind any of His actions is hidden from the believer, then he should still adhere to this principle, and not think of his Lord in bad terms.
The specific answer is that the wisdom behind the creation of these things is to manifest the perfect nature of Allaah’s creation and control of all things. Even though the created things are so many, He provides for them all. He also tests people by means of these (dangerous creatures), rewards those who are afflicted by them and makes manifest the bravery of those who kill them. By creating them, He tests His slaves’ faith and certainty: the believer accepts the matter and submits, whilst the doubter says, “What is the point of Allaah creating this?!” He also demonstrates the weakness and incapability of man, whereby he suffers pain and sickness because of a creature which is far smaller than he is. One of the scholars was asked about the wisdom behind the creation of flies. He said, so that Allaah may humiliate the noses of the tyrants with them. Because of the existence of harmful creatures, it becomes apparent how great is the blessing in the creation of beneficial things, as it is said that contrast with the opposite demonstrates the nature of things. The study of medicine has demonstrated that many beneficial drugs are derived from the venom of snakes and the like. Glory be to the One Who has created benefits in things that to all outward appearances are harmful. Moreover, many of these dangerous animals are food for other creatures which are beneficial, and this forms the ecological cycle in the environments where Allaah has created them.
But the Muslim has to believe that everything that Allaah does is good, and that there is no pure evil in what He creates. In everything that He creates there has to be some aspect of good, even if it is hidden from us, as is the case with the creation of Iblees, who is the head of evil. But there is wisdom and a purpose behind his creation, for Allaah tests His creatures by means of him, to distinguish the obedient from the disobedient, those who strive from those who are negligent, the people of Paradise from the people of Hell.
We ask Allaah to bless us with strength of faith and understanding of our religion. May Allaah bless our Prophet Muhammad and grant him peace.