We have: traps that are placed pretty randomly into underground locations, mineshafts and dwellings that pop up in some pretty deep locations, a large, fleshy wall with two eyes and one mouth you summon by throwing a voodoo doll of the first human you meet, and somebody apparently rebuilding large monsters using machines. That means nothing good could have happened to those who took up underground.
There are werewolves at night-time. What are werewolves? People. You're on an island with few people to help you, and you're really killing them.
The Clentaminator can be this, just think about it, you're a creature living in the Crimson/Corruption biome, then some random adventurer just destroys your home, turning the grass blue, the stone white, and then you see all the people you used to know being slaughtered in front of you...
If one looks closely, the Crimson background features what appears to be mountain sized bones, such as a skull and a ribcage.◊ That and the addition of ichor, seems to be evidence as to how the Crimson came to be.
You can stop pre-Hardmode Corruption from spreading by planting sunflowers. In real life, folks living in Japan plant sunflowers to absorb radiation from the nuclear plant that was flooded in the tsunami.
They did it prior to that in Ukraine to help purify the soil contaminated by the Chernobyl disaster.
Sunflowers have long been planted around the edges of fields, especially among the Pennsylvania Dutch community, to ward against evil.