A confession to murder is valid and the heir of the victim is entitled to Qisas, but it is recommended to ask him to let him go

Sahih Muslim · Book 28 · Hadith 1680 a

حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ الْعَنْبَرِيُّ، حَدَّثَنَا أَبِي، حَدَّثَنَا أَبُو يُونُسَ، عَنْ سِمَاكِ بْنِ، حَرْبٍ أَنَّ عَلْقَمَةَ بْنَ وَائِلٍ، حَدَّثَهُ أَنَّ أَبَاهُ حَدَّثَهُ قَالَ إِنِّي لَقَاعِدٌ مَعَ النَّبِيِّ صلى الله عليه وسلم إِذْ جَاءَ رَجُلٌ يَقُودُ آخَرَ بِنِسْعَةٍ فَقَالَ يَا رَسُولَ اللَّهِ هَذَا قَتَلَ أَخِي ‏.‏ فَقَالَ رَسُولُ …

'Alqama b. Wa'il reported on the authority of his-father: While I was sitting in the company of Allah's Messenger (ﷺ), a person came there dragging another one with the help of a strap and said: Allah's Messenger, this m…