Muqatil bin Hayyan (may Allah have mercy on him) said concerning the saying of Allah Almighty "Then whoever is pardoned something by his brother" [Surah al-Baqarah: 178]: When someone kills another deliberately, and the guardian of the slain pardons him, does not take retaliation and accepts the diyah, then "let there be a following up in a fair manner" [Surah al-Baqarah: 178], meaning he should demand the diyah in a good way, and "payment to him with excellence" [Surah al-Baqarah: 178], meaning the diyah should be paid in a good way and with excellence. He said that for the people of the Torah only retaliation was obligatory. Then he narrated the previous hadith in full.
And he (Muqatil, may Allah have mercy on him) says in explaining "Whoever transgresses after that will have a painful punishment" [Surah al-Baqarah: 178]: Whoever takes the diyah and also kills, for him is this punishment. That is, if someone kills another and the killer speaks to the guardians of the slain and gets them to agree to diyah, and when he comes to pay the diyah the guardian of the slain kills him and says: "I accepted the diyah only so that he would come to me and I could kill him," then this is: "Whoever transgresses after that will have a painful punishment" [Surah al-Baqarah: 178], that is, after accepting the diyah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 16033