The wrath of The Lamb is righteous. (Rev.16) But we also see mankind’s constant rebellion as they “blasphemed” their Maker, and “did not repent.”
God’s judgement is absolutely righteous because man’s stubbornness is so hard and unyielding. The Father has done everything He can do to save mankind. He sent His only Son, Jesus Christ, to die for mankind. And yet, even when they experience the results of their sin, some of them will not repent. Nothing God does can bring them to repentance, so they must be sentenced to everlasting punishment. There is no other righteous thing God can do.