بَلَىٰ مَن كَسَبَ سَيِّئَةً وَأَحَاطَتْ بِهِ خَطِيئَتُهُ فَأُولَٰئِكَ أَصْحَابُ النَّارِ ۖ هُمْ فِيهَا خَالِدُونَ
Balaa man kasaba sayyi'atanw wa ahaatath bihee khateee'atuhoo fa ulaaa'ika as-haabun naari hum feehaa khaalidoon
On the contrary — whoever earns evil and is encompassed by their sin, those are the companions of the fire; there they will remain.
Aḥāṭat, the verb for sin encompassing a person, is the word for an enclosing wall or a surrounding army, so the image is of being ringed with no opening left. That is what the sentence turns on: the completeness of the wrong rather than its weight. The verse opens with a particle of correction that answers the claim in the line before rather than beginning a new subject.