Where's the justice?

[FallentoReason]

Two non-Christians are going about their life. They have a dispute that ends in one being killed by the other. The one left alive in his/her later years becomes a Christian.

Now, according to theology, that person is going to heaven. The one who was killed is eternally getting punished. Where's the justice? How does the killer go unpunished like that?

Why should there be justice in the Christian myth?

Because like many other claims, this one gets thrown around alot. So here I am asking for the justification.