Actually, he mentioned that when they tried to take down Gotham before, the combination of their building Wayne tower and their deaths sparked the citizens of Gotham to come out of the depression that The League of Shadows caused.
The whole cause of the depression is revealed to be The League of Shadows' doing:
Ra's Al Ghul: Of course. Over the ages, our weapons have grown more sophisticated. With Gotham, we tried a new one: Economics. But we underestimated certain of Gotham's citizens... such as your parents. Gunned down by one of the very people they were trying to help. Create enough hunger and everyone becomes a criminal.
Their deaths galvanized the city into saving itself... and Gotham has limped on ever since. We are back to finish the job. And this time no misguided idealists will get in the way. Like your father, you lack the courage to do all that is necessary. If someone stands in the way of true justice... you simply walk up behind them and stab them in the heart.
However, you could say he indirectly caused their deaths by making the depression therefore making Chill desperate enough to rob and shoot a rich couple in a back alley. Ra's created his downfall that fateful night.
While clearly capable of such acts, I doubt Ra's Al Ghul would leave such an important task of making sure the Waynes are dead in the incapable hands of a street rat like Joe Chill, and would have assassinated them himself, as well as Bruce.