The goal of British involvement in the opium trade in China was definitely profit rather than corrupting the Chinese people. Chinese nationalists and anti-opium crusaders -did- talk about a foreign “duhua zhengce” (policy to poison) carried out by the British and later by the Japanese but reality is, as always, a bit complicated.
/u/EnclavedMicrostate has some good stuff here
First, when the Indian-China opium trade started to take off (around 1800-1820) nobody in Britain or anywhere else though of opium as an addictive substance that would corrupt a society in the modern, Nancy Reagan sense. That was an idea that was developed in the late 19th century in the West. Opium was a medicinal and also an intoxicant (sort of like alcohol) but the modern idea of a plague of illegal drugs would only be developed later (partially drawing on the model of China). The trade was of course quite profitable for the British, but moral condemnation came later. The Anglo-Oriental Society for the Suppression of the Opium Trade was only founded in 1874.
As new ideas about opium and drugs began to spread (often spread by Christians and influenced by the temperance movement) the British state began to move away from the trade. By 1890 or so the trade was largely in the hands of marginal imperial groups (Armenians, Jews) and in 1907 the British signed an agreement to end imports of Indian opium into China. Even after this however, selling opium (mostly to Chinese) was a big part of the revenue of colonies like Singapore and Hong Kong, and colonial officials resisted the eventual end of the trade both for direct revenue reasons and because they assumed that attracting cheap Chinese labor required opium.
Even in the 20th century, when the Japanese became the chief foreign providers of drugs to China (opium, but also morphine and heroin) the chief goal was revenue. The Japanese began eliminating the trade in both Taiwan and Manchuria because of worries about colonial productivity, i.e. the direct revenue from drugs was not worth the lost productivity.
So while both the British and Japanese were accused of having a cunning plan to weaken the Chinese people, they were really just in it for the money.
There is a big literature on this.
Brook, Timothy, and Bob Tadashi Wakabayashi. Opium Regimes: China, Britain, and Japan, 1839–1952. Berkeley: University of California Press, 2000.
McMahon, Keith. The Fall of the God of Money: Opium Smoking in Nineteenth-Century China. Lanham, MD: Rowman & Littlefield, 2002.
Padwa, Howard. Social Poison: The Culture and Politics of Opiate Control in Britain and France, 1821–1926. Baltimore,: Johns Hopkins University Press, 2012. League of Nations. Records of Commission of Enquiry into the Control of Opium-Smoking in the Far East. Geneva, Switzerland: League of Nations, 1930.
Smith, Norman. Intoxicating Manchuria: alcohol, opium, and culture in China’s northeast. Vancouver: UBC Press, 2012.
Trocki, Carl A. Opium, Empire, and the Global Political Economy: A Study of the Asian Opium Trade, 1750–1950. New York: Routledge, 1999.