One of the earliest full-length horror films, and one of the USA's first of note, this D.W. Griffith feature, while enjoyable, is a mixed bag.
The main problem is that this is a moderately meandering, melodramatic morality tale. Murder is bad, and thou shalt not kill, and all that rot, but it doesn't make for an amazing story. Still, for the most part, things worked out okay.
The first 55 minutes were all solid, with a few seemingly-less necessary portions, but after a certain point, things felt as they were dragging. It picked up again at the end, with a twist of sorts (though really, it makes sense in the context of the story), and I rather enjoyed the conclusion.
Henry B. Walthall did a fantastic job as a young man on the edge of sanity - you could tell that toward the end, his character was drenched in uncertainness. Walthall, overall, did a fantastic job. His uncle, played by Spottiswoode Aitken, was memorable also, though I wish we saw a bit more of him. While no one else stood out to me, everyone played their roles fine.
Making many references to Edgar Allan Poe (constantly quoting 'Annabel Lee', and alluding to both The Cask of Amontillado and The Tell-Tale Heart), portions of The Avenging Conscience do come across as perhaps darker than you would think. The score, at times jovial, at times almost frantic, really helped to make some scenes more suspenseful.
The Avenging Conscience: or, 'Thou Shalt Not Kill' may not come across to many as a real horror movie, despite both murder and revenge from beyond the grave, because of the amount of romantic drama, but I'd urge any fan of horror to still give it a shot. It's far from perfect, and not even close to the best silent horror flick, but it's still solid despite the flaws. 7.5 (rounded down to 7/10 to fit site's format).