Definitely sounds like a weasel. If you have your coop as well protected as you say, a coon likely wouldn't be able to get in. A weasel will be able to get through holes that most people would never believe.
Peanuts will never attract a weasel, or most other predators for that matter. Weasels thrive on fresh blood, and usually attack the heads of birds, since there is a huge amount of blood flow through the head/neck area.
There is another possibility, and that is that a mink might be doing the killing. Mink are also able to get through tiny holes. However, a mink most likely would have killed all your birds. Mink are notorious for killing just for the fun of it.
I would try using a smelly type bait, such as a can of tuna, or something similar. If you have a trapper in your area, he likely would have some mink lure, and that should also help to attract a weasel. Good luck!
