Explain why Syme believes the narrowing of the range of thought is necessary.

Syme is the most intelligent person Winston knows but paradoxically the most zealous supporter of the Party. The ideology of the Party, as in all totalitarian regimes, demands unquestioning acceptance from its members, and the more limited people's thoughts are, the less likely they are to have "heretical" thoughts and rebel against the Party. Hence Syme's desire for narrowing the range of thought of which the citizens of Oceania are capable.
During lunch in the canteen at the Ministry, Syme explains to Winston his work on the Newspeak dictionary and the obvious delight he takes in it, saying, "the destruction of words is a beautiful thing." The Party's intention is to reduce the vocabulary of the English language to such an extent that eventually, anti-Party thought will become impossible because there will be no words available in which to express it. Orwell's own theory, expressed in the appendix of 1984 and in his earlier essay "Politics and the English Language," is that there is a direct connection between clarity of thought and the way in which people use words. If a politician or a writer, for example, uses unclear, sloppy language, it facilitates his ability to engage in deception and to make lies sound convincing. Similarly, if (as in the dystopia of 1984) the vocabulary is cut down to an absolute minimum of words required for communication, then people will not even be able to think anything not dictated to them by the Party.
One of the unique features of the English language as it exists today is the size of its vocabulary, since English, probably more than any other language, has undergone a process over the centuries of borrowing an immense number of words from other languages. Though Syme doesn't mention this fact—which he may not even be aware of because the citizens of Oceania are forbidden to learn foreign languages—he states that, for instance, all the synonyms for the word "good" are useless and should just be replaced by prefixes to show the degree of "goodness"—plusgood, doubleplusgood, and so on. Even antonyms are seen as superfluous, so that "bad" is replaced by "ungood." Insofar as thought is dependent upon words—a point Orwell makes clear in the appendix—the intention of the Party is not merely to limit thought and make heretical thoughts impossible but to eliminate the ability to express emotion, and thus to turn people into robot-like beings.
Syme himself, because he understands this purpose so clearly, is, as Winston realizes, a danger to the Party in spite of his unquestioning devotion to it. His intelligence makes him the opposite of the unthinking machines into which the Party is trying to transform the masses. So Winston is not surprised when Syme vanishes, when all references to his existence have been eliminated. Syme has been "vaporized." One wonders if this fate is worse than what ultimately happens to Winston, who is imprisoned and "reeducated." Winston survives but is turned into the zombie-like being the Party intends everyone to become.