If you have ever purchased the game then using the ROM is not illegal
Hmmm, I'm not entirely sure of the voracity of this statement I'm afraid Klouud. By using a torrent site in the first place you are uploading the ROM while you download it, meaning that you could be (and probably would be) unwittingly distributing copyrighted material to people who didn't buy the original game. At least, I'm pretty sure that's how the copyright holder would see it.
I think the only way you could legally use a copied ROM is if you copied the ROM yourself from your own, bought copy of the game and then only used it on your own devices. Otherwise there's undoubtedly a chance it could be construed as bootlegging in some form or another. To be honest, it's a minefield. If you are using copied anything, the copyright holder can usually find some element of legal wrongdoing. I'd steer clear, but then, I don't live very dangerously.