Using AAct violates Microsoft's Licensing Agreement. It is considered software piracy, which is illegal in most jurisdictions.

Configuration is another aspect. Maybe the tool has a configuration file to adjust anti-aliasing settings. The user would need to edit that with a text editor. Also, compatibility issues—if the game doesn't support AA at high levels, the tool might not work. Testing in a non-competitive game first is probably a good idea.

Legal Implications of Using Activator Tools for Windows - YTU