chris ryves set up find boston bombers thread after the 2013 bombings .
within hours , millions of users had taken to subreddit to identify bomber .
they analysed photos and videos of spectators at marathon on april 15 .
slung false accusations at those wearing backpacks or acting strangely .
claims led to sunil tripathi , 22 , being wrongly identified as the suspect .
frenzy only halted after dzhokhar and tamerlan tsarnaev were named .
now , ryves has told of regrets about reddit thread in new documentary .
in film , the thread , he says the subreddit ` became almost its own beast '