The Humanoid appears in the lighthouse out of the weird hole that formed, the source of The Shimmer. It’s a human shape with an abalone shell, almost. It seems tall and skinny at first, but morphs into the human shape. It’s hard to gauge what this alien life form wants other than infiltrating the human species.
I’m not sure it directly kills anyone.
It alters its DNA to mirror a human, eventually taking the human’s place.
The movie doesn’t give any context to the whys of what’s happening.