It's nothing personal - just some poor JW (probably elderly or infirm) trying to convince God he/she's working hard enough for Him not to destroy them at Armageddon. My 91 year old mother has written some of those letters when she was unable to make it to the KH for "telephone work". Don't be surprised if you get a call. The letters are given to pioneers to either drop off at the not at homes in the territory, or mail (guess who pays for the stamps). Using these letters to specifically target DNCs wouldn't surprise me a bit. *sigh*
Meantime, don't let them get under your skin.

Hugs, Cee Cee