My personal thoughts on the matter are that it's not that there is some kind of being or guardian of the Laws. The Laws and the consequence of breaking them are natural phenomena like when you throw a stone, gravity will act upon it and it will eventually fall down. There is no Spirit of the Higgs Boson sentiently acting to make it fall, it's a feature of the universe we live in.
For the Dresden Files, the Laws and what happens when you brake them is a consequence of two fundamental properties of the Universe being forced to work against each other. First off, magic is the energy of creation left over after the universe came into existence, this has been stated at least ones by Dresden directly if not multiple times. Second, mortal Free Will is special, while other creatures have more or less leeway in their actions humans are the only beings in the whole of existence that can say that they are truly of Free Will.
Now let's look at the Seven Laws of Magic through the lens of Free Will, shall we?
"Thou Shalt Not Kill" - easiest, killing someone prevents them from executing their Free Will anymore
"Thou Shalt Not Transform Others" - forcing someone out of their human body that holds Free Will, limits it in an obvious way (it might even suggest that Free Will is something biologically/metaphysical to the human body)
"Thou Shalt Not Invade the Mind of Another" - directly influencing and breaking someones Free Will
"Thou Shalt Not Enthrall Another" - indirectly influencing and breaking someone's Free Will
"Thou Shalt Not Reach Beyond the Borders of Life" - bringing back human souls, capable of Free Will, and binding it to your will
"Thou Shalt Not Swim Against the Currents of Time" - going back in time to unmake Free Will decisions
"Thou Shalt Not Open the Outer Gates" - that one is just there so that no one brakes the Universe by accident (looking at you Dresden)
So my theory goes, that the Taint from practising black magic comes from the crash of two fundamental elements of the universe. You are using magic (the force of creation) to unmake/damage Free Will (a building block of creation) the negative feedback from that damages reality in the form of your psyche giving the black magic corruption effect.
So it's a natural phenomenon, no gods or convents necessary. Just a sideproduct of how the Dresdenverse is set up.