How do we differentiate between the fear of Allah and having a negative assumption about Him, especially when considering Al-Hasan Al-Basri's saying, "We laugh, and we do not know; perhaps Allah has observed some of our deeds and said: 'I will not accept anything from you'"?

The difference between the fear of Allah that is praiseworthy in Islamic law and the blameworthy ill-conjecture about Him is as follows: Fear [of Allah] is a reverence for Allah that necessitates abandoning disobedience to Him and refraining from negligence in His obedience. This is praiseworthy and commanded. As for ill-conjecture about Allah, it is despairing of His mercy, which is blameworthy in Islamic law because it leads to despondency. The limit of praiseworthy fear is that which restrains one from disobeying Allah. Anything beyond that is unnecessary and leads to despair and despondency.