It seems to me the more meaningful question would be what does it say about their state of mind if they do. I see forgiving others as more to do with me than them. Forgiving doesn't mean forgetting. When I forgive I am letting it go and letting go the power it held over me. But I'm not forgetting it like it didn't happen. I'm not erasing the memory. I am erasing the feeling the memory gives me.
With that said, examples like Hitler aren't that useful because grudges are personal and usually involve people we know personally. That's where the real test occurs.