
Kevin C. answered 04/08/19
Theology Geek: Greek, Latin, Proofreading, Writing, ACT-SAT-GRE
Fascinating example! I think any way you have it, the sentence has a typo. The answer depends on what the subject is. There are a couple of ways of looking at this sentence. Is the subject "magistrate" taking two verbs here: "had gone" and "bent"? Independent clauses are not separated by commas when the subject takes two verbs. The rule of thumb: if it were to stand alone as a sentence, would something be missing? In the example you provide,
This was at a moment when the magistrate, overcome with tiredness,
had gone down into the garden of his house
and, dark, bent beneath some implacable thought, like Tarquin cutting the heads off the tallest poppies with his cane, M. de Villefort was knocking down the long, dying stems of the hollyhocks that rose on either side of the path like ghosts of those flowers that had been so brilliant in the seasons that had passed away.
But if that were the case, the comma after "thought" should instead be a semicolon.
Another alternative, one I think more likely, is that you are right there should be a comma because "bent" is not a verb but part of a participial phrase functioning as an adjective modifying Villefort.
This was at a moment when the magistrate, overcome with tiredness,
had gone down into the garden of his house,
and, dark, bent beneath some implacable thought, like Tarquin cutting the heads off the tallest poppies with his cane,
M. de Villefort was knocking down the long, dying stems of the hollyhocks that rose on either side of the path like ghosts of those flowers that had been so brilliant in the seasons that had passed away.
The giveaway is the word bent. If bent is active voice, then the first is the correct reading with no comma. If bent is passive voice, then it's clearly a participle and the second is correct.