Is the injunction in the Benedictine rule “Deinde non occidere” interpreted to forbid the killing of brutes, or only humans?
The quotation comes from the beginning of Chapter 4 of the Rule, where St. Benedict lists the "instruments of good works". The list begins by quoting several of the commandments:
In the first place, to love the Lord God with the whole heart, the whole soul and the whole strength.
Then one’s neighbour as if oneself.
Then, not to kill.
Not to commit adultery.
Not to steal.
Not to covet.
Not to utter false witness.
So it seems that these "instruments" were intended to be understood just as the commandments were ordinarily understood. In particular, this commandment forbids killing people, not lower animals.
The Rule also allows the sick and infirm to eat flesh meat (Chapter 36), which would seem incompatible with a general injunction against killing animals.
The "instruments of good works" that are mentioned in the fourth Chapter of the Rule Of St. Benedict are to St. Benedict the tools that every monk should possess in order to attain eternal life.
Chapter 4 lists 73 "tools for good work", "tools of the spiritual craft" for the "workshop" that is "the enclosure of the monastery and the stability in the community". These are essentially the duties of every Christian and are mainly Scriptural either in letter or in spirit. - Rule of Saint Benedict
The injunction not to kill should be taken as one of the ten commandments found in the books of Exodus and Deuteronomy of our bibles.
There is no injunction that monks may not slaughter animals for human consumption. The monks of Cluny used to slaughter pigs, which in turn they gave away to the poor. The monks of Cluny were themselves vegetarians!
As a former Benedictine, I can assure you that monks slaughter many of their domestic animals for food. At least those communities that that have farms. In the monastery that I was in, the monks slaughtered pigs, lambs and chickens at the appropriate time. We were not vegetarians.