Its not just about what the ethical choice is.
The key is obtained by other writing down the key from a legitimate license sticker OR by using a keygen.
So the key you purchase is either ALREDY registered and being used by a different user, or best case scenario it is a key that has not been sold yet.
When the rightful owner of that key goes to install it on his computer it will fail, they will call Microsoft, rightful owner has the sticker and proof of purchase and thus their install of widnows will get activated and yours will be deactivated.