That’s not entirely the case. They aren’t little vampires, but they do jump on and bite the head/neck to kill. A chicken is just too big for a mink to carry off most of the time, so they will go for the high calorie bits first, often the brain. They probably intend to come back for more but that’s not before people find them, so it just looks like they don’t eat much or that they’re “killing for fun.” Mink do the same thing. I used to trap muskrats for fur and once in a while I would have a mink run down my whole trapline and just eat the heads off all the muskrats I caught. Maybe it’s a mustelid thing.