Grace allows us to be forgiven for anything we've done.
It allows us to always make Jesus our foundation upon which we build (1 Cor. 3).
Grace seems to unfair to us as humans (Jesus parable of the field workers: Luke 20)
I have heard people say "well, I wouldn't have gotten so angry with that person if I saw some consideration on their part or some sign that they were sorry".
But, in point of fact that statement puts us as a human in the judgement seat - and we don't get that seat, we don't get to judge. All we can do is apply forgiveness and grace the way that it has been applied to us.
If the person we apply it to continues to sin then grace will be applied even more fully (Romans 5:20) as long as the individual isn't taking advantage of grace to sin purposefully.
It can be frustrating and even unbearable to forgive someone for something very very wicked that has caused pain in our life. But, if we are to receive God's forgiveness then we must do it. (forgiving someone doesnt' mean we make them our best friend or have a relationship with them - it means to me that we let go of the anger and need to avenge ourselves on that person).
When we ask ourselves, what would Jesus do, we usually come to the conclusion that he would forgive the person, perhaps with some timely advice. What we want to be careful of is that we don't judge someone and withhold our forgivenss based on our judgement.