An EvilMonopoly is a monopoly that exploits its position excessively. Their product or service is sold at a much higher price than the total costs of the corporation would allow.
One can roughly see this from the revenue/turnover percentage.
Usually this is the case with monopolies, because they exist to avoid the competition of a free market of producers that compete for the customers by offering a large variety of products and qualities.
"Evil" shouldn't be understood in a moralic sense, but in wiki tradition, like VotingIsEvil, LoginsAreEvil, Wiki:BenchmarksAreEvil, Wiki:ButtonsAreEvil, Wiki:ConstructorsAreEvil, Wiki:CopyrightsAreEvil, Wiki:GoogleIsEvil, and dozens more... of course, monopolies are monopolies and free markets are free markets. Neither more nor less.