Allah states that even if the people were commanded to commit what they were prohibited from doing, most of them would not submit to this command, for their wicked nature is such that they dispute orders. Allah has complete knowledge of what has not occured, and how it would be if and when it did occur. This is why Allah said,

﴿وَلَوْ أَنَّا كَتَبْنَا عَلَيْهِمْ أَنِ اقْتُلُواْ أَنفُسَكُمْ﴾

(And if We had ordered them (saying), "Kill yourselves (i.e. the innnocent ones kill the guilty ones)) until the end of the Ayah. This is why Allah said,

﴿وَلَوْ أَنَّهُمْ فَعَلُواْ مَا يُوعَظُونَ بِهِ﴾

(but if they had done what they were told,) meaning, if they do what they were commanded and refrain from what they were prohibited,

﴿لَكَانَ خَيْراً لَّهُمْ﴾

(it would have been better for them,) than disobeying the command and committing the prohibition,

﴿وَأَشَدَّ تَثْبِيتاً﴾

(and would have strengthened their conviction), stronger Tasdiq (conviction of faith), according to As-Suddi.

﴿وَإِذاً لاّتَيْنَـهُمْ مِّن لَّدُنَّـآ﴾

(And indeed We should then have bestowed upon them from Ladunna) from Us,

﴿أَجْراً عَظِيماً﴾

(A great reward), Paradise,

﴿وَلَهَدَيْنَـهُمْ صِرَطاً مُّسْتَقِيماً ﴾

(And indeed We should have guided them to the straight way.) in this life and the Hereafter.