When someone harms us, they create the cause of their own suffering. They do this by strengthening habits that imprison them in a cycle of pain and confusion. It's not that we are responsible for what someone else does, and certainly not that we should feel guilty. But when they harm us, we unintentionally become the means of their undoing. Had they looked on us with loving-kindness, however, we'd be the cause of their gathering virtue.
What I find helpful in this teaching is that what's true for them is also true for me. The way I regard those who hurt me today will affect how I experience the world in the future. In any encounter, we have a choice: we can strengthen our resentment or our understanding and empathy. We can widen the gap between ourselves and others or lessen it.