You must own a physical or digital copy of the game and "dump" the files from your own console to your PC to stay within legal boundaries [3].
If you want to play Mario Kart on your computer, here is the safest way to go about it: Use an Emulator Mario Kart 8 PC Download Free
Emulating modern consoles requires a decent CPU and dedicated graphics card to run smoothly at high resolutions. You must own a physical or digital copy
Find the needed to run the game smoothly. Mario Kart 8 PC Download Free
Suggest (like Forza or Hot Wheels Unleashed) that offer a similar experience.
The only way to play Mario Kart 8 on a PC is through emulation.