first-born of ... poor - Hebraism, for the most abject poor; the first-born being the foremost of the family. Compare “first-born of death” (Job 18:13), for the most fatal death. The Jews, heretofore exposed to Philistine invasions and alarms, shall be in safety. Compare Psa 72:4, “Children of the needy,” expressing those “needy in condition.”
feed - image from a flock feeding in safety.
root - radical destruction.
He shall slay - Jehovah shall. The change of person, “He” after “I,” is a common Hebraism.