30. 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 Ps 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.