Aloysius O'Hare is the main Antagonist of the movie.
After The Once-ler ruined the environment by cutting down the last of the trees for his Thneed business, there were no more trees left to clean the extremely polluted air, leaving people with lung problems because there was no clear air to breathe. O'Hare exploited people's need for clean air by starting his own company: O'Hare's Air. Using machines that polluted the outside environment even more, he filtered the dirty air into clean air and charged people for jugs of O'Hare Air, making himself rich.
In order to assure that people would continue to need his air and pay him for it he created a monopoly and eventually a Plutocracy in which he, being the wealthiest and therefore most influential person in town, sealed off the town from the rest of the world and held the townsfolk in blissful ignorance of the sorry state of the earth's environment. None of them were aware that the process of filtering SOME air for them to be able to breathe easy meant further polluting the rest of the air outside of the city.
And that's the way he liked it because it meant more money for him, so it was no surprise when he went after Ted Wiggins upon finding out the boy had a Truffula Tree Seed. O'Hare intended to destroy it to ensure his Plutocracy permanently.