Many people feared the creation of monopolies and trusts because of the negative impact it would have on the economy. Monopolies tend to drive a product's prices up, people did not want to see businesses become so powerful that it controlled the government.
People were against monopolies and trusts because they would put the smaller competitors out of business. The trusts and monopolies could also raise prices as high as they wanted because they'd have no competition.
Most people oppose monopolies and trust because they destroy the potential for competition. If a business or entity exist with no competition it drastically reduces opportunities for other entrepreneurs and also limits choice.