what’s the rhetorical figure called of mentioning things by “omitting” them?
Preterition/praeterition
Qimmik mentioned that Cicero loved to use the relative superlative: how do you tell the difference between a relative and absolute if they have the same form and often mean essentially the same thing? (e.g. “a most excellent man” vs. “the most-excellent man” in English)
I think I wrote that Cicero liked absolute superlatives. You’ll notice how frequently he uses them, to the point where to my ear he sounds pompous and orotund. Context is the only way to tell them apart (and they aren’t really that dissimilar)–whether or not a class of individuals, objects, etc. is specified or implied as a basis for the comparison or whether the superlative is just an intensifier.
Quo etiam maiore sunt isti odio supplicioque digni qui non solum vestris domiciliis atque tectis sed etiam deorum templis atque delubris sunt funestos ac nefarios ignis inferre conati.
What are the subject and predicate of the main clause of this sentence? This is the first question you need to ask.
The subject is isti.
The predicate is sunt . . . digni.
maiore odio supplicioque: what is its case and function in the sentence? It’s ablative, and it’s the complement of digni, which normally takes an ablative complement; and etiam, “even,” modifies the comparative maiore: “they are worthy of even greater hatred and punishment”. Cicero has emphasized etiam maiore by fronting it and setting it off from the nouns it modifies by hyperbaton, interposing the less important words sunt isti.
Then we come to a relative clause, introduced by qui, which is masculine plural nominative and modifies isti, specifying who the subject of the main clause, “they,” refers to: qui . . . sunt . . . conati. Sunt is the only finite verb form in this clause, so it must be (or be part of) the main verb of the clause, and conati is masculine plural nominative, so it must refer to the subject isti. Sunt conati fits together as a periphrastic perfect: “they tried” (not “try”).
What did they try to do? You would expect an infinitive complement of conati, and, sure enough, we find an infinitive verb with its obvious object: ignis inferre: “set fire[s]”
funestos ac nefarios, which are masculine accusative plural, can’t be predicate adjectives agreeing with nominative isti (as you thought). They must modify and agree with masculine accusative plural ignis: “deadly and nefarious fires” (if I thought hard enough I might be able to think of a more elegant English translation, but you get the picture).
With in-ferre we expect a dative complement, and we have vestris domiciliis atque tectis . . . deorum templis atque delubris. Two neatly balanced pairs, each a hendiadys.
The rest falls into place:
“Therefore, those men who tried to set deadly and nefarious fires not only to your dwellings and houses but also to the temples and shrines of the gods deserve even greater hatred and punishment.”
I think you could have done better on this sentence if you had tried to apply grammatical analysis a little more rigorously. When you come to a sentence you don’t understand, first identify the subject and predicate of the main clause, which should have an finite indicative verb, then do the same for any subordinate clauses and identify their functions, and then fit the whole together. If you do this enough, you’ll get better at it, and you’ll do this instinctively for more and more sentences, which will allow you to dispense with this “decoding” process most of the time.