The reward for forgiveness
Because of that We ordained for the Children of Israel that if anyone killed a person not in retaliation of murder, or [and] to spread mischief in the land – it would be as if he killed all mankind, and if anyone saved a life, it would be as if he saved the life of all mankind. And indeed, there came to them Our Messengers with clear proofs, evidences, and signs, even then after that many of them continued to exceed the limits [e.g. by doing oppression unjustly and exceeding beyond the limits set by Allâh (swt) by committing the major sins] in the land!
The aggrieved relatives of the victims have a very strong incentive to forgive the murderer.
And so judge [you O Muhammad (saws)] between them by what Allâh [swt] has revealed and follow not their vain desires, but beware of them lest they turn you [O Muhammad (saws)] far away from some of that which Allâh [swt] has sent down to you. And if they turn away, then know that Allâh’s Will is to punish them for some sins of theirs. And truly, most of men are Fâsiqûn [rebellious and disobedient to Allâh (swt)].