|"Desires are never fully satisfied:If you are not content with the halal, no amount of haram will fill you up."|
A sin alone will never make you a Kāfir..
For example, let’s say you gain billions of dollars through interest, but you still acknowledge that it is a sin. It will be a major sin on your part, but you are still a Muslim.
But if you believe interest is Halāl, EVEN if you don’t have any money gained through interest, you have committed major Kufr because you have made something Halāl that Allāh has clearly forbidden in the Qur'an!
Acting upon the evil deed is only a sin, justifying it is Kufr.