I, too, still hold hatred towards OW (although it's diminishing) and I've never hated anyone in my life. As everyone else has suggested, it's a great idea to write that letter to release some of your anger and destroy it. I can tell you from personal experience in having contacted OW a couple of times that it did no good. You would just be wasting your time. Most OP have no conscience. If they did they never would have gotten involved with a married person in the first place.
Me47 H46 S13 M16 Piecing since May/09
"Life is 10% what happens to us and 90% how we react to it." Lou Holtz