Whether the evildoer is Christian or not is irrelevant. And while attempting to vainly cover up the crime immediately after it happens is wrong, it's also a very common impulse (goodness knows I've seen it in my kids, even down to when one was 10 months old!) That doesn't tell us anything about the regret and reflection that takes place over time.
The sad thing about cases like this is that the unforgiving attitude ends up hurting the victims family more than the evildoer. They can't let it go and it just eats at them.
“Letting go” does not require forgiveness.
God does not even forgive, without repentance. How can you begin to justify your stance? It is not biblical, at all.