Vengeance and judgment belong to the Lord
Hebrews quotes two declarations from the Song of Moses while warning against rejecting the Son of God and insulting the Spirit of grace.
Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses. How much more severely do you think one deserves to be punished who has trampled on the Son of God, profaned the blood of the covenant that sanctified him, and insulted the Spirit of grace? For we know Him who said, “Vengeance is Mine; I will repay,” and again, “The Lord will judge His people.” It is a fearful thing to fall into the hands of the living God.
Deuteronomy 32:35 says that vengeance and repayment belong to God, and verse 36 says that the LORD will judge His people; Hebrews 10:30 quotes both declarations in succession.