COMMENTARY on 4:91
A. Yusuf Ali:

Translation:
Others you will find that wish to gain your confidence as well as that of their people: Every time they are sent back 610 to temptation, they succumb thereto: if they withdraw not from you nor give you (guarantees) of peace besides restraining their hands, seize them and slay them wherever you get them: In their case We have provided you with a clear argument against them.
Commentary:

610  As opposed to the two classes of deserters to whom clemency may be shown, there is a class which is treacherous and dangerous and cannot be left alone. They try to win your confidence, and are all the time in the confidence of the enemy. Every time they get a chance, they succumb to the temptation of double-dealing. The best way of dealing with them is to treat them as open enemies. Keep them not in your midst. If they give you guarantees of peace and do not actually fight against you, well and good. If not, they are deserters actively fighting in the ranks of the enemy. They have openly given you proof, and you can fairly seize and slay them in war as deserters and enemies.

 

Muhammad Asad:

Translation:
You will find [that there are] others who would like to be safe from you as well as safe from their own folk, [but who,] whenever they are faced anew with temptation to evil, plunge into it headlong. 112 Hence, if they do not let you be, and do not offer you peace, and do not stay their hands, seize them and slay them whenever you come upon them: for it is against these that We have clearly empowered you [to make war]. 113
Commentary:
112  Lit., "whenever they are returned to temptation (fitnah), they are thrown back into it" or thrown headlong into it".
113  Lit., "that We have given you clear authority (sultan)" - a solemn reiteration of the ordinance which permits war only in self-defence (cf. 2:190 ff. as well as the corresponding notes).