To show that GOD does the avenging.
Can you avenge yourself?
No. Only GOD can avenge Me. GOD is the Avenger.
Romans 12:19 (Amplified Bible)
12:19 Beloved, never avenge yourselves, but leave the way open for [God’s] wrath; for it is written, Vengeance is Mine, I will repay (requite), says the Lord.