Mice & Rats will eat/nibble on bait in box traps.
Sounds like a Weasel (or Mink).
It's very hard to catch a Weasel in a Hen House with box traps. They are focused on fresh, live chickens and their natural behavior to kill prey takes over.
As far as killing more than one chicken, in hen house...
I offen heard about this over the years and witnessed once.
I only have my opinion and oberservation... There are many chickens running, flopping, flying, clucking so much confusion and mayhem, I think the Weasel makes a kill, then see another easy kill and another and another and another.
I saw this same behavior from a Red Fox in a hen house. Killed 11 took 5 on two trips.
Maybe predators get caught in the monment when they have the chance, maybe it's their nature. Someday I may know.
muttbuster, I have better success trapping Weasels at chicken coops with 'blind sets', outside next to wall(s) of coop.