Whoa, is this in the Dark Knight Trilogy cannon?
I don't think Nolan was thinking about Green Arrow when writing his triology but Arrow writers definetely thinking about TDK triology.
Can't find the quote but they basically said that Ghul was the man who trained Merlyn, and that they envisioned him taking the same training as Bruce Wayne. Where Wayne refused to kill the criminal, Merlyn did.
As far as I'm aware from wikipedia and things, Merlyn was always a member of the league of assassins although I don't know if they ever referenced him with R'as or if they ever appeared together. So it makes some sense the plots are similar.