Just because a tool is hosted on GitHub doesn't mean it is safe. Using an unauthorized activator comes with significant baggage:
Unlike "cracks" found on shady websites, GitHub allows users to inspect the code (though many users don't). github windows 8.1 activator
Since Windows 8.1 is no longer supported by Microsoft, using a third-party tool leaves you entirely on your own if something breaks. How to Stay Safe While Exploring GitHub Just because a tool is hosted on GitHub