The Host is a relatively new movie with Saoirse Ronan that was written by the same person as the Twilight series (I found this out just now). I never watched any of Twilight, but this Host movie was creepy.
It was creepy because it reminded me of a reverse demon possession.
Usually, in the Bible and in movies, when people are possessed by a demon, they try to destroy their host. For example, "Lord [Jesus], have pity on my son, for he is a [possessed] lunatic and suffers severely; often he falls into fire, and often into water." (Here's another example (Legion).)
There was a reversal of this tendency in The Host. The alien "souls" who were possessing the humans were stopping them from being violent to other humans and the Earth.
Why is this a bad thing? (Why don't you tell me in the comments below?) Perhaps it is because taking away someone's freewill is against God's prime directive (God's will).
(Speaking of prime directive, the Trill on Star Trek (Dax et al.; Dr. Beverly's Trill) had hosts who freely submitted to being used as hosts. So, the same objection can't really be raised in that case.)