I didn't get it...(SPOILER)

The thing in the well was a hungry monster who is able to control the minds of humans and force them to help it get food. It appears that it can only control one person at a time though. So after he killed the previous "puppet" the monster started to control the main character to get what he wanted. It is referred to as "Lovecraftian" because, in Lovecraft's fiction, many of the monsters have strong telepathic powers.