قَالَ
...
(Allah) said,
i.e., threatening and warning Iblis.
...
هَذَا صِرَاطٌ عَلَيَّ مُسْتَقِيمٌ
This is the way which will lead straight to Me.
means, `all of you will return to Me, and I will reward or punish you according to your deeds: if they are good then I will reward you, and if they are bad then I will punish you.'
This is like the Ayah:
إِنَّ رَبَّكَ لَبِالْمِرْصَادِ
Verily, your Lord is ever watchful. (89:14)
وَعَلَى اللَّهِ قَصْدُ السَّبِيلِ
And it is up to Allah to show the right way. (16:9)