I've found black widows (Latrodectus mactans) are good at getting rid of cockroaches and other harmful pests, so I just leave them be. Brown recluses (Loxosceles reclusa), on the other hand, are dangerous, for they nest in lcothing and in beds and other areas where they come into contact very frequently. They also have three different kinds of venom, and one type shuts down the kidneys. They should be killed, as they are very dangerous, unlike the black widows (L. mactans, hasselti, hesperus, bishopi, geometricus, and variolus), which stay mainly outside, and have venom that isn't deadly. So, it's best just to leave some spiders alone, because cockroaches are very well known to adapt to chemical sprays and not be affected by them, thus allowing them to live in human habitats and spread disease. They have yet to develope an immunity to black widow or any other spider venom. So, spiders are good for pest control.