If you have a powerful company, they will inevitably overpower ever other company in every market. While it's the "ultimate goal" of capitalism to own it all, it causes one company to have all the power and would quickly place one man in global control of everything, including food stocks, farms, oil, and necessities of life. If you defy such corporation, should you be shot by one of their paid mercenaries? Who would stop them from having such force? If you don't buy their products they could send a hit squad after you. Does any of this sound evil to you? Yes, I realize I went to the far extreme, but total domination of any market could lead to that. Seriously. If GM owned all the car companies in the world, they could control all the shipping, all the businesses (deny selling to one business in favor of their new upstart shipper?) Once they control that market, they turn their sites on another, steel mills. Buy them all up, own the railroads, construction, etc.
Monopolies are detrimental to EVERYBODY, except the ones in charge (and arguably, even the ones in charge.) Gamers, programmers, businesses... everybody.
In the case of why Microsoft is specifically evil? What would happen if Balmer ran the company into the ground? Let's say Microsoft closed shop, took the keys, and denied anyone the ability to use their source code unless they paid them 100 trillion dollars. Should we be forced to comply? I mean, Microsoft built that code form the ground up (hah, yeah ... not.) They should have full rights to determine how it's used or if it's used? What would the businesses that run Windows exclusively do for patches and support? The economy would fall, the Great Depression wouldn't hold a candle.