But you thought that the Messenger and the believers would never return (48:12)

Almuntakhab Fi Tafsir Alquran Alkarim

In effect you people thought that the Messenger together with the faithful believers should be caught in the jaws of death and never would they return back safe to their people. This mental anticipation allured your hearts by hopes of relief, and you hugged this evil thought to your hearts and you were a morally depraved people

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(48:12) (But the truth is not what you say.) You had imagined that the Messenger and the believers would never return to their families, and this notion was embellished in your hearts. *23 You harboured an evil thought, and you are an immensely evil people.' *24

But you thought that the Messenger and the believers would never return meaning

*23) That is,"You were delighted to think that you had saved yourselves from the danger into which the Messenger and his believing supporters were going. You thought you had done so by dint of your great wisdom; and you also did not feel any compunction in rejoicing at the thought that the Messenger and the believers would not return alive from their expedition. You did not feel uneasy in spite of your claim to the Faith but were pleased to think that you did not put yourselves in the danger by accompanying the Messenger. "
*24) The word ba- ir (pl.bur in the original) has two meanings: (1) A sinful, perverted and evil-minded person, who is incapable of doing anything good; and (2) one who is doomed to an evil end, who is following the path of destruction,
 

But you thought that the Messenger and the believers would never return meaning in Urdu

(مگر اصل بات وہ نہیں ہے جو تم کہہ رہے ہو) بلکہ تم نے یوں سمجھا کہ رسول اور مومنین اپنے گھر والوں میں ہرگز پلٹ کر نہ آ سکیں گے اور یہ خیال تمہارے دلوں کو بہت بھلا لگا اور تم نے بہت برے گمان کیے اور تم سخت بد باطن لوگ ہو"