Charles Ellicott Commentary Malachi 3:5

"And I will come near to you to judgment; and I will be a swift witness against the sorcerers, and against the adulterers, and against the false swearers, and against those that oppress the hireling in his wages, the widow, and the fatherless, and that turn aside the sojourner [from his right], and fear not me, saith Jehovah of hosts." — Malachi 3:5 (ASV)

All these crimes were explicitly forbidden by the Law. Sorcery (Exodus 22:18), adultery (Exodus 20:14; Leviticus 20:10; Deuteronomy 22:22), false-swearing (Leviticus 19:12), defrauding, or withholding of wages (Leviticus 19:13; Deuteronomy 24:14–15), oppressing the widow and orphan (Exodus 22:22–24), doing injustice to a stranger (Deuteronomy 24:17; Deuteronomy 27:19). (Zechariah 8:16–17).