NOOOOOOOOOOOOOOOOOO just get a new console before you ruin all your discs! Don't even attempt to continue trying to ruin your discs.

should i digitally download my game to prevent any scratched occurring on my game discs?

You know this isn't the first time someone has reported on having disk scratches on their mario kart 8 discs, also don't use your Wii U vertically anymore if Nintendo gives you a replacement... and I'm pretty sure there's a knife or something hidden in your Wii U for it to make that bad of a scratch (Also, check your other discs to see how they are).

actually, my game disc's top side is slightly torn, not scratched. so, do you think it is still safe for me to play my games? 

It seems like your Wii U is damaged and is scratching the disks. You should call Nintendo.

Can i still play any wii u game disc?

Your wii u seems to be scratching your disc's as noted above, I wouldn't put another disc in there until after contacting nintendo immediately.

Is your wii u flat or vertical? Does it make weird noises when the disc was in? just curious..but contact nintendo asap

it is vertical

This indicates that something inside your Wii U is scratching the heck out of the top of your discs. You need to contact Nintendo about getting it fixed if you're still under warranty...they should repair your Wii U and give you another copy of Mario Kart 8, imo.

If i dont fix my wii u, will my game discs still damage my console?

I have played Mario Kart 8 and when I ejected the game disc out of my wii u console, there are some scratches on the disc image. Does this indicate that my game disc is scratched and will damage my wii u?