Or rather, the horror of reality. Poe was great with the horrors of the mind, but when it came to the horrors of human nature, Dickens had him beat.
The following is from my review of Barnaby Rudge. As it's my own text, I'm not going to even blockquote it:
Let me open with a quote from the book:
"On the skull of one drunken lad -- not twenty, by his looks -- who lay upon the ground with a bottle to his mouth, the lead from the roof came streaming down in a shower of liquid fire, white hot; melting his head like wax."
You think Edgar Allen Poe to be the king of horror? Meet the author who far surpasses him in making flesh creep and cringe, in this book more than most. In Barnaby Rudge, Dickens portrays the Gordon Riots, a violent anti-Catholic outburst in London in 1780, supposedly started by the opposition by Lord Gordon to some acts in Parliament which would've relieved some of the burdens of Catholic citizens: the right to directly inherit property and the right to educate their children in England. As usual, Dickens ignores the political pressures, likely brought about by the French allying themselves with the Americans in their colonial war, which would've stirred up suspicions of Anglo-Catholics being in league with the French.
In any case, I can see why this book has never matched the popularity of "A Tale of Two Cities", as there is no possibility for heroes in scenes of mob violence. Even those who refuse to cooperate with the rioters can do little more than resist. Locksmith Gabriel Varden, being forcibly brought to Newgate prison, refuses to pick the lock for the masses of rowdy men intending to free their comrades and anyone else locked inside; however, his resistance means little as the mob decides to burn down the door instead. Mr. Haredale is often defiant, but he can do little more than beat a retreat when faced by hundreds of belligerent men. None of this "'Tis a far, far better thing..." kind of statement which can be made.
Ignore the plot lines. They are cursory, even for Dickens. They are there merely to set up the characters and their own personal motivations so one can see what happens to them when all order dissolves. I could see why the English of Dickens' day may have not wanted to read this book -- it was far too scary, especially as Dickens portraying the fuel to the rioters' fire being the primal urges to destruction and looting. Though there was then (in 1841) a better policing force and better prisons, there still was the unbearable poverty and dissipation alive in London. Then, as now, there were plenty ripe to take advantage of any opportunity to set fire to the town. Think the actions in here too far-fetched for even these times? Consider the fires, destruction, and looting that start in any modern city, once rioters have been given an opening.
People are crushed underfoot, men are consumed by the very fires they had set, and decent people stand by because they are powerless in the face of the massive wave of violence. Dickens shirks not one detail. If you're thinking of a quaint Victorian period piece, for crying out loud, don't read this book! Go read Pickwick Papers or Old Curiosity Shop. However, if you enjoy this kind of disaster, by all means, read Barnaby Rudge (or might I suggest Hard Times, which has some pretty horrific deaths).
Back from the review, I want to point out that the Gordon Riots occurred while the American Revolution was going on: the act that they were rioting against was passed in 1778, the riots themselves occurred in 1780, and the Revolution didn't end until Yorktown in 1783.
France had joined on the American side in 1778 (when the Papists Act had been passed). France, obviously, was a Catholic nation. So I suppose one could see the Papists Act of 1778 might be one way to shore up British Catholic support for the crown, and keep the bits of the aristocracy that had remained Catholic from supporting the French (and Spanish, though Spain was on the wane at this point.) Amusingly, when something similar had been passed to apply to Canada, so the French Canadian Catholics wouldn't go rogue, in 1774, the American colonists saw this as an offense.
For all that anti-Catholic jawboning, though, it's not like religion played a huge role in the Revolution... at least on the American side. But one could see where some of the British Protestants might be a bit wary of what France might be doing around these riots (there was an intimation that France itself instigated the anti-Catholic riots, so that the French could invade England itself..... hey, conspiracy theories abound in all times and places.)