Tafsir Ibn Abbas: Al-Jaathiya, Ayah 21

Or am has the significance of the rhetorical hamza of denial do those who have perpetrated evil acts such as disbelief and disobedience suppose that We shall treat them as those who believe and perform righteous deeds equally sawā’an is the predicate in their life and in their death? mahyāhum wa-mamātuhum this clause constitutes the subject and a supplement; the sentence itself is a substitution for the kāf of ka’lladhīna ‘as those’ and both suffixed pronouns -hum refer to the disbelievers. The meaning is do they suppose that in the Hereafter We will assign them a good fate like believers that they will enjoy a life of plenty one equal to their life in this world as when they said to the believers ‘Surely if we were to be resurrected we will be given the same good things that you will be given!’ God exalted be He says in accordance with His denial implicit in the rhetorical interrogative hamza How evil is that judgement which they make! In other words it will not be so for in the Hereafter they will be suffering in the chastisement in contrast to their lives in this world; the believers on the other hand will be delighting in the Reward in the Hereafter because of their righteous deeds in this world such as their observance of prayers their alms-giving fasting and otherwise the mā ‘that … which’ relates to the verbal action that is to say ba’isa hukman hukmuhum hādhā ‘awful as a judgement is this judgement of theirs’.