O man!
What has made you careless about your Lord, the Most Generous?
This is a threat. It is not an attempt to get a reply as some people mistakenly think. They consider it as if the Most Generous is asking them so that they will say,
"His honor deceived him (or made him careless of his Lord)."
rather the meaning of this Ayah is,
"O Son of Adam!
What has deceived you from your Lord, the Most Generous -- meaning the Most Great -- so that you went forth disobeying Him, and you met Him with that which was unbefitting."
This is similar to what has been reported in the Hadith,
يَقُولُ اللهُ تَعَالَى يَوْمَ الْقِيَامَةِ
يَا ابْنَ ادَمَ مَا غَرَّكَ بِي
يَا ابْنَ ادَمَ مَاذَا أَجَبْتَ الْمُرْسَلِينَ
Allah will say on the Day of Judgement;
"O Son of Adam!
What has deceived you concerning Me?
O Son of Adam!
What was your response to the Messengers?"
Al-Baghawi mentioned that Al-Kalbi and Muqatil said,
"This Ayah was revealed about Al-Aswad bin Shariq who struck the Prophet and he was not punished in retaliation. So Allah revealed,
مَا غَرَّكَ بِرَبِّكَ الْكَرِيمِ
(What has made you careless about your Lord, the Most Generous?)"
Then Allah said,
الَّذِي خَلَقَكَ فَسَوَّاكَ فَعَدَلَكَ