Prometheus was the one who killed Cayden James' son as part of his insane Gambit Roulette to destroy Oliver's life.
- Or maybe not directly him, but either Evelyn or Talia, who will turn out to be alive.
http://tvtropes.org/pmwiki/pmwiki.php/WMG/ArrowS6E11WeFall
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Prometheus was the one who killed Cayden James' son as part of his insane Gambit Roulette to destroy Oliver's life.