Maybe this card was the Zodiac Killer's response. The Zodiac Killer was fond of tragedy, mailing a blood soaked piece of Paul Stine's shirt either in this letter or the Bus Bomb Letter the following day, according to the San Francisco Chronicle October 12th 1969 article, entitled 'I've Killed Seven the Zodiac Claims.' In the article it states "As proof the letters were from the true Zodiac and not an imposter, the maps murderer enclosed another portion of the piece of shirt he tore off the body of San Francisco taxicab driver Paul Stine, his victim last October 11th." A clear statement that the murder of Paul Stine was indeed the work of the Zodiac Killer and was 'written in blood'.
Well maybe this was the Zodiac Killer's second response, contained within the same 'Dripping Pen card,' and delivered with a sense of irony: "I get aufully lonely when I am ignored, so lonely I could do my Thing."