Lyric discussion by Ziing 

Cover art for Gemini lyrics by Slayer

Yeah, I thought the song was about the Zodiac Killer as well, especially from the lines

Gemini master of demise Your death is my salvation To a kingdom mine My lord is my light The master of darkness Your death is my salvation To a kingdom mine

The Zodiac Killer revealed in the letters he wrote to the police department that he was killing people so that he would have slaves to serve him in the afterlife, and that every slave he killed would help serve him