"It might be why it sucks, but if vigilante justice, or in this case looking at photos and pointing out who had backpacks resulted in the capture of the true bomber more quickly, than the unfortunate consequences for the people temporarily named by a subreddit who's existence would be hard to locate without media attention is just not significant enough to outweigh the possibility of them figuring it out before they got on a flight etc.
A terrorist will argue that sacrifice is worth it to achieve their goals, which they see as justice in their terms.
We have to be better than that, or there is literally no point bothering. We just do their work for them. People say Islamic terrorists hate American freedoms. So how come Americans give them up so easily citing the likes of Islamic terrorism? It just goes round and round, bomb, less freedom, bomb, less freedom, until you have less freedom than they have.
The logic you refer to is "the ends justify the means"... but I assure you that I considered both in my statement. There are definitely arguments to be made that say I am wrong.
Sounds vaguely like how the US justifies torture.