Spiders are soft bodied animals and can be eaten and consequently have many enemies.
Nematodes (round worms) and mites are known parasites of spiders. The mites can easily be spotted on the spider as red dots.

Also birds, lizards, geckos, scorpions, centipedes catch spiders from the ground or out of their web.
There are wasps specialized in spider hunting, the spider wasps (Pompilidae), use the captured spider to lay their eggs in. The larvae consume fom inside the paralyzed spider.
There are also spider flies (Acroceridae) that attack spiders and deposit the young fly larvae to a spider's leg. These larvae penetrates the book lungs and kills the spider consequently.
Ants can sometimes been seen dragging a killed spider.
The praying mantis, a great killer, eats any insect or spider it runs into, even another praying mantis that happens to be on it's knees.

                                Wasp with spider.                                                                    Mantis, Mantis religiosa
 

The greatest threat to the spiders is the spiders themselves. When times are hard and food is scarce, it becomes hard for the spider to overlook it's next of kin.

A familiar spider in our house is the daddy-longleg, Pholcus phalangioides.She is a great spider killer. In springtime, she is one of the last living spiders in our house. Every other insect or spider is consumed by herm during the winter. In the end when famine strikes, they even kill their next of kin.

Daddy-longleg, Pholcus langioides
 

Also in the family Mimetidae and Ero are specialized spider killers. Ero attacks a spider by biting it in one of its legs. Then it retreats and waits at a safe distance until the bitten spider is paralyzed in a few seconds. Then it returns to suck it empty.

We humans, have a bad habit of disturbing the natural world for our own selfish ends. In doing so we destroy many habitats in which the spiders live. Insecticides in the agricultural activities wipe out the whole populations of insects and spiders. Many species of spiders are on a red list nowadays and are in danger of becoming extinct. Tarantulas from South-America have become rare because many of them are caught and sold as pet animal.

Law with the only exception of studying purposes "forbids killing of spiders".