WHY was Jesus cricified?

[John V]

You're missing the context. I mean "flat out forgiven" not "requires a blood sacrifice, then you get forgiveness".

If Jesus tells us to forgive people without some sacrifice or payment given to us, why does God need a sacrifice to forgive everyone's sins?

Matthew 18:22 tells us to forgive people 77 times. I was always taught that it's not a literal cap, but telling people to keep forgiving others.

He doesn't tell us to flat out forgive. He tells us to forgive in recognition that He is forgiving us. You chose a good passage, but stopped reading too soon. Read vv. 23-35.

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The problem is that Christians appear to define justice in this instance as whatever god thinks is just.

Why is that a problem? An atheist defines justice as whatever he thinks is just. You don't seem to see that as problematic. Even if he uses a framework such as utilitarianism or the harm principle, the choice of framework is just his preference.