Well, nicotine kind of is one of the most harmful things about cigarettes. During the curing and processing of tobacco leaves, nicotine and other alkaloids partially react with nitrate to form nitrosamines ("Tobacco-Specific Nitrosamines" or "TSNAs"), that contribute greatly to tobacco's carcinogenicity. They're not the only carcinogens in tobacco / tobacco smoke, of course, but they are important. (This is, afaik, not relevant for pure nicotine.)