There is no biblical prohibition against killing insects, bugs, spiders, or other such creatures. The Old Testament Law even went so far as to separate out the clean insects that the Israelites could eat from the unclean insects that they couldn’t—eating bugs is one way to get rid of them! “All flying insects that walk on all fours are to be regarded as unclean by you. There are, however, some flying insects that walk on all fours that you may eat: those that have jointed legs for hopping on the ground. Of these you may eat any kind of locust, katydid, cricket or grasshopper. But all other flying insects that have four legs you are to regard as unclean” (Leviticus 11:20–23).
Of course, most of us today are not looking to the world of insects as a food source. When we kill spiders or insects, it is often because they are pests in our homes. There is absolutely nothing wrong with keeping a clean and safe home by ridding one’s house of ants, silverfish, centipedes, scorpions, brown recluse spiders, cockroaches, bedbugs, moths, fruit flies, house flies, horse flies, mosquitoes, etc., etc., etc.
Some people choose to rid their homes of pests “humanely”; that is, they would gently catch the black widow spider crawling across their pillow, keep it alive, and deposit it a safe distance from their house. That is fine. Others would rather swat the spider, stomp on it a few times to make sure it’s dead, and then burn the pillow. That’s fine, too. It’s a matter of conscience. The biblical prohibition “thou shalt not kill” (Exodus 20:13, KJV) refers to premeditated murder of a human being, not squashing the spider on your pillow.
We should make a distinction, however, between ridding one’s home of pests and simply killing spiders and insects for no good reason. God has a purpose for “every creeping thing that creepeth upon the earth” (Genesis 1:26, KJV). The spiders and insects are God’s creation, and He owns them: “I know every bird in the mountains, and the insects in the fields are mine” (Psalm 50:11). To treat any of God’s creatures, no matter how small, with wanton cruelty is sin. Focusing a magnifying glass on an ant hill just to see the ants suffer, sprinkling salt on a garden slug just to see it writhe, purposely stepping on any type of bug on the sidewalk just for the morbid satisfaction of it—these are not the actions of a God-fearing person.
Everyone at some point or another unintentionally kills spiders and insects and other less visible creatures simply by walking down the street. This cannot be helped and is not a sin. It’s also not a sin to keep oneself and one’s family safe and healthy—go ahead and swat the fly, kill the tick, and smash the venomous spider. It’s not sinful to protect one’s property, either—go ahead and exterminate the termites and spray for those weevils. A case can also be made for killing spiders and insects in the interest of learning—go ahead and help your child pin the bugs on the board for her science class. But, out of respect for the Creator of all things, we should not participate in the unnecessary, gratuitous killing of spiders and insects in the wild.