Tafsir of Ash-Shams 91:12

(The word) "When" (idh) is connected to "denied" (kadhdhabat) or to "transgression" (taghwa). "Was sent forth" (inba'atha) is the intransitive form of "to send" (ba'athahu), meaning he dispatched himself.

The meaning is: when he went forth to slaughter the she-camel, the most wretched of them—that is, the most wretched of (the tribe of) Thamud, who was Qudar ibn Salif. Or, it could refer to him and those who undertook the slaughter with him among the wretched ones; (the number of them being) two, according to what Al-Farra' said, or more. For the superlative form (af'al at-tafdeel), when annexed to a definite noun, can apply to a single individual, a group, the masculine, or the feminine.

Their wretchedness was deemed greater than that of others because they directly committed the slaughter, while all (of the tribe) shared in the consent to it; and due to other wickednesses known only to Allah, the Exalted, which they possessed, being greater than the wickednesses of those other than them.