Because you are downloading an executable file (.exe) that you must run as administrator to "install" the game, you are handing the keys to your computer to a stranger.
"99% Compression" is mathematically impossible for games already using compressed asset formats (like Unreal Engine .pak files). If a torrent claims to compress Red Dead Redemption 2 from 150GB to 3GB, it is either a virus or a fake download link. Highly Compressed Pc Games Torrent