The clear intent of this is to be a completion of what the magicians admonished Fir`awn with. They warned him of the vengeance of Allah and His eternal and everlasting punishment. They also encouraged him to seek Allah's eternal and endless reward. They said,
إِنَّهُ مَن يَأْتِ رَبَّهُ مُجْرِماً
(Verily, whoever comes to his Lord as a criminal,) This means, whoever meets Allah on the Day of Judgment while being a criminal.
Surely whoever comes to his Lord as a sinner, for him there is Jahannam .:.. And whoever comes to Him as a believer ... for such people there are the highest ranks.... (20:74-76)
These words which refer to the Islamic beliefs and the hereafter have been spoken by the magician who only recently became Muslims and who had not yet been instructed about the principles and the obligations of the new faith.
It was the result of their brief association with Sayyidna Musa (علیہ السلام) that Allah, in an instant, revealed to them the mysteries of their new faith so that they gave scarce heed to Pharaoh's threats. Thus with the adoption of the religion of Allah they attained a state of sainthood which is given to others after long and assiduous exertions. Sayyidna Ibn ` Abbas and ` Ubaid Ibn ` Umair ؓ said it was a divine marvel that the magicians who started the day as infidels became saints before it ended. (Ibn Kathir)
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 76–78
What is meant by ‘becoming a criminal or a delinquent’? Delinquency is that state to which a man descends when he has a sign from God shining brightly before his eyes, but fails to learn a lesson from it. Truth is revealed to him in the language of reason, but he ignores it. He is not capable of pulling himself out of the grip of worldly forces and material considerations to the point of admitting the Truth. In the life hereafter there is the sternest punishment for such people. The troubles of this world, howsoever great, are necessarily limited in their severity by the very fact of being bound to come to an end—sooner or later—with death. But the hereafter is a place from which man, beset by unspeakable afflictions, will find it impossible to run away, for there, he will not even have death ahead of him to put an end to his torment. Paradise is for one who purifies himself and purification consists of giving up a life of carelessness and neglect, and adopting a prudent way of living. The aspirant to Paradise abstains from all things that come in the way of Truth. He rids himself of the hindrance of any worldly considerations obstructing his path. He crushes the base desires of his self when they arise. If feelings of false pride and the urge to transgress raise their heads within him, he stifles them and buries them in his innermost self. Such are men of true faith. In this world their faith takes the shape of a garden of pious deeds. In the life hereafter, it will be returned to them in the shape of heaven’s eternally flourishing garden.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.