Assalam o alaikum,
Sir, I've a confusion. And I really want to get rid outta it. I know Allah is most just and he knows everything best. But sometimes I think that what if someone is really cruel and he has spent his entire life in doing bad things, and even he kills someone but then he realizes and ask forgiveness to Allah, and of course Allah is the most Merciful, and he will forgive, then how the person who has been killed or hurt will be justified? I mean he has suffered a lot, and in case he's killed he can't return back to this world but if Allah forgives the person who has hurt him or killed him then the person who was subjected will gain justice in what way? I hope you understand my question. And I'm sorry for my wordings if they were wrong. May Allah forgive me."
On the day of judgement Allah will surely do justice. Allah will give options for both paries for reconciliation and will settle the matter as per their choice.
[Quran 49:10 The Believers are but a single Brotherhood: So make peace and reconciliation between your two (contending) brothers; and fear Allah (God), that you may receive Mercy.