This was a violation of the Law which was given to Moses for the nation of Israel. This can be made explicit. Alternate translation: "by eating meat without draining the blood first as required by the law" (See: [[rc://en/ta/man/jit/figs-explicit]])
Saul is accusing his whole army of acting unfaithfully though this is a generalization because not every soldier acted unfaithfully. (See: [[rc://en/ta/man/jit/figs-hyperbole]])