Forgiveness: a necessary evil?

I have been thinking alot about the nature of forgiveness. The previous post and our discussion in class on Monday have further provoked my thoughts. What is the nature of forgiveness? Can someone ever truly forgive even as the memory lingers? We can see forgiveness as foundational in such religions as Buddhism and the Amish sect of Christianity. The fact that the Amish forgave Carl Roberts so quickly and seemingly effortlessly will never cease to amaze me. Is there really a power of forgiveness? I like to think yes, someone can truly forgive an action and thus heal from the past, but I am not so sure. It is awfully hard to forgive someone who has viciously murdered you family. What about someone who orchestrated a genocide such as Hitler? Can we ever really forgive him? Is there a certain degree of transgression which we can forgive? Where do we draw the line?