Commentary
Why was Sayyidna Musa (علیہ السلام) afraid?
The words إِنَّنَا نَخَافُ (We are afraid) signifies that here Sayyidna Musa (علیہ السلام) and Sayyidna Harun (علیہ السلام) expressed twofold fear before Allah. The first fear is pointed out by the word أَن يَفْرُطَ which means to cross all boundaries. So the meaning of the sentence is 'perhaps the Pharaoh would attack even before listening to us'. The other fear is mentioned with the words أَن يَطْغَىٰ - which means that Sayyidna Musa (علیہ السلام) was apprehensive that the Pharaoh might, after listening to his demands, become so refractory and perverse that he might speak about Allah in disrespectful terms.
It will be recalled that when prophethood was bestowed upon Sayyidna Musa (علیہ السلام) ، he had prayed to Allah to depute Sayyidna Harun (علیہ السلام) also to assist him in his mission, and Allah had accepted his prayer. At the same time Allah informed him سَنَشُدُّ عَضُدَكَ بِأَخِيكَ وَنَجْعَلُ لَكُمَا سُلْطَانًا فَلَا يَصِلُونَ إِلَيْكُمَا ' ("We make. your arm stronger through (the help of) your brother and will make for you the upper hand so they will not reach you (with any harm) "-28:35) and that whatever else he asked for was granted to him (قَدْ أُوتِيتَ سُؤْلَكَ يَا مُوسَىٰ ) "You have been granted your request 0 Musa" - 20:36. Among the things granted to him was شرح صدر (heart at peace) which means that he was made immune to fear. So, when he had received such firm assurances from Allah, how does one explain the fear expressed by him in this verse. One answer to this question is that the promise of victory and security from harm is rather vague in that victory may refer to success in debate with the Pharaoh and his men. It can also be argued that victory could come only if the Egyptians heard his arguments and reasoning, and saw his miracles but there was this possibility also that they might attack him before he had laid his arguments before them. Besides, شرح صدر does not mean elimination of natural fear.
The second point is that fear of fearsome things is a natural instinct and even prophets are not free from it in spite of their complete faith in the promises of Allah. Sayyidna Musa (علیہ السلام) himself was afraid to pick up his staff when it turned into a snake therefore Allah re-assured him with the word لَا تَخَف (do not be afraid). On all such occasions Allah removed their fears by propitious tidings. Three other verses: فَخَرَجَ مِنْهَا خَائِفًا يَتَرَقَّبُ (So, he went out of it (the city) in a state of fear, waiting (for what comes next) - 28:21) and فَأَصْبَحَ فِي الْمَدِينَةِ خَائِفًا (Then, next morning he was fearful, waiting [ for what comes next ] - 28:18) and فَأَوْجَسَ فِي نَفْسِهِ خِيفَةً مُّوسَىٰ (So Musa concealed some fear in his heart - 20:67) can also be quoted expressing the same fear. It was this natural fear which persuaded the Holy Prophet ﷺ to migrate to Madinah and some of his Companions ؓ to migrate first to Abbysinia and later to Madinah. In the Battle of Ahzab the Muslims were so overawed by the strength of their enemies that they dug a trench as a protective measure even though Allah had promised them victory on numerous occasions. The truth is that while they did not have the least doubt about their ultimate victory, the fear which they felt was the result of a spontaneous human impulse in the face of danger, and the prophets being human are not immune to this impulse.