Question:
My son killed a spider and now feels remorseful. He wants to know if the spider had a soul. I would also like to know which animals have souls.
Answer:
The truth is that not only spiders, but everything, even an amoeba, has a soul. The soul is the embodiment of every created thing's purpose, the reason G‑d created it.
Therefore, we should be careful not to destroy anything without purpose. However, in general we are not prohibited from destroying living organisms when the destruction can benefit us. For example, we are permitted to exterminate mosquitoes so that they don’t cause us discomfort, or to chop down trees to use for building.
The concept of murder applies only to the human being who was created in G‑d’s image. While all of creation needs to be valued and treated respectfully, only human life has absolute value independent of circumstances.1
Please see the article, What is a Soul? and our additional article on The Soul.
Rabbi Shmary Brownstein
The Arizal was careful never to kill any living creature, even if it was a mere insect and was a nuisance. This was likewise the custom of the Rebbe Rashab. Some state that one should never directly kill a creature, even if it causes pain, unless it can cause real injury, such as a snake, and one is rather to indirectly cause them to die, such as through poison. It is thus a Midas Chassidus not to directly kill a living creature with one’s hands even if it is a nuisance, and is hence permitted to do so from the letter of the law. [If however the indirect killing will cause the animal more pain, than seemingly it better to kill it directly. The Gemara states that Rebbe Yehuda Hanaasi experienced suffering due to a certain incident that involved Tzaar Baalei Chaim, even though it was permitted according to Halacha and it was then removed after he showed mercy to animals.