Yes, it's illegal and considered piracy, even if you have the game already.
If you rip a game properly that's you making a backup copy of your disc, and you can know it's done properly.
On the other hand, downloading one from the internet is transferring a copy, which is not allowed, and anyways you don't know if it's been modified or not without hash checks.
Really? I thought in North America, if you own the game already, you can download a digital version off the internet for free, but that just means the physical copy can't be used until you delete the digital version. At least that's how it is in canada. If I own the physical, I can download the digital without any trouble coming my way at all.