Some of the evidence that Edwards was the Zodiac killer is briefly as follows:
1. A survivor of a Zodiac attack said the Zodiac mentioned being in Deer Lodge prison Montana, and other biographical details consistent with Edwards. Edwards was in Deer Lodge from 1956-1959. (It’s Me, p. 30). This survivor also gave a description consistent with Edwards. The Zodiac was wearing a black hood and had the Zodiac’s signature cross and circle on his front.
2. The Zodiac sent a message, which when decoded, mentions being reborn in paradise and “all that I have killed will become my slaves”.
Edwards’ autobiography describes an inmate in Deer Lodge prison, stating “He believed that anyone he killed would be his slave in the next life.” (It’s me p. 40)
3. The message had 18 symbols left over, which when decoded (in 2010) read “I’m Edward E. Vexed I.Q. Hath You Greater Than I”.
4. Edwards left a very similar coded confession “I killed Laci Peterson”, signed “Edward E”. See https://www.facebook.com/notes/scott-peterson-is-innocent/edward-w-edwards-left-a-signed-confession-i-killed-laci-peterson/427291067432419
A documentary on the Zodiac Killer: