RMS doesn't say Mono (a .Net implementation) is evil. He says the fundamental .Net system is controlled by Microsoft and any licensees who depend upon .Net are subject to the goodwill of Microsoft to live an upstanding life. Microsoft has been well documented as being highly aggressive and destroying (or otherwise crippling) competitors to its business. As a result, it would be risky for the free software community to make Mono a fundamental part of a free system and it would be better to avoid Mono/.Net altogether.