Part of Zed's Lore reads:
The old man implored Zed to enter the temple, destroy the box, and lead his followers to balance. The dark ninja followed the master inside. Moments later, the assembled ninjas heard Zed cry out in pain. Mysteriously, he emerged unscathed, and threw the severed head of the master at Shen's feet. Screaming in rage, Zed commanded his followers to slaughter the master's students and seize the box.
This to me sounds like the master (the old man) betrayed Zed and attempted to slay him, only to slay one of his shadows instead (zed can make shadow images of himself), at which point Zed slays the master in revenge. Am I interpreting this correctly?
Zed's Lore in full: http://na.leagueoflegends.com/news/zed-master-shadows-revealed