It is absolute basic animal instinct, some people are confused about the habits of animals like foxes that will kill every hen in a hen house. They are not killing for fun, they are thinking ahead. A wild animal doesn't know where its next meal is coming from, and if it finds a glut of prey in one place, it makes sense to kill as many as it can while they're available. Given enough time, it would carry off and cache every chicken it killed.
I'm the same, on the rare occasions I have to help the missus with the shopping at Tesco's, I totally load up the trolley with everything I can get my hands on, beer, curry, non own label stuff, you just don't know when you will get the chance again.