The Laws of Magic and the Soul
Breaking the first four laws always stains the soul. Breaking the the 5th and 7th laws violate the natural order of things, but can in some circumstances also stain the soul. The White Council always assumes that violating the 5th and 7th has gone far enough to stain the soul, or that it will eventually. Breaking the 6th law doesn't stain the soul but causes so many problems that it might as well as far as the council is concerned. Breaking the 6th law always breaks the natural order of things.
Who is subject to the Laws?
This question really has two parts, whose soul gets marked by Lawbreaking acts, and whom do the Wardens hunt down and punish.
Creatures with Souls
Any creature with a soul is subject to the metaphysical laws Only things born as mortals have a soul. Souls can be lost, given up, or subverted to the will of another being. In game terms those things which do have souls are Mortal Practitioners, Mortals, Scions (The first Merlin was a half-demon, scions are mortal and have souls), Changelings (before choosing to be faeries or after choosing to be mortals), People of Faith (they also answerable to a more stringent set of rules), Classic Werewolves, Hexen Wolves, Lycanthropes, White Court Vampires and Virgins, Red Court Infected and The Summer and Winter Knights. There might also be some strange things which count as having souls, intelligent zombies (ie a wizard cursed to reside in an undead body) but those exceptions are for a GM to rule on.
Creatures without Souls
The following creatures do NOT have souls and are not subject to the metaphysical laws: Demons, Dragons, Faeries, Ghosts, Ghouls, Gods, Hecatean Hags, Old Ones, Outsiders, Spirits, Full Red Court Vampires, Black Court Vampires, Warped Animals, Valkyries, Zombies, and Wolfweres.
A few things deserve mention:
In the end the Unseelie Accords, the Council itself and politics determine who are subject to the Council. A few creatures whom at first glance might be under the Council's authority may or may not be. A good example is Dresden himself once he becomes the Winter Knight. Becoming the Winter Knight doesn't mean he loses his soul, or leaves the White Council, it just puts another player in the game, the Winter Court. If Dresden breaks a Law it stains his soul, and the Wardens probably come after him, unless the Winter Court steps in and protects him and causes a political fight.
Outside the Council's Authority
Not being subject to the Councils authority doesn't mean a character can break the laws, it just means its not the Council's job to enforce them, unless of course the Wardens think they can get away with it. A character also cant just claim immunity from the Council. Think about the factions as countries metaphor, a person must be the citizen of some country, even if they don't like the country. In order to cease being a citizen of one country a person must become a citizen of another. Yes I know there are some exceptions, they are homeless and living in airports, that is not a game option.
Only those characters which meet the requirements of having a soul, and being under the Council's authority (even if its questionable) can take Lawbreaker stunts. In game terms the cost of the stunt (-1 refresh) for a +1 across the board bonus is too cheap, especially when stacked with other specializations and refinements unless the Wardens are looking for the character. Doing something to completely remove a character from the Councils authority (like becoming a Denarians) voids any Lawbreaker stunts the character might have.
Getting screwed without the bonus
A character doesn't have to actually get credit for the Lawbreaker stunt for the Council to come after them. The Council can do it even if the character is not guilty or cant take the stunt for some other reason. If a
Specific Notes on the Laws
Taking life applies to anything with a soul. That isn't to say killing something without a soul doesn't stain, its just not covered by the Laws of Magic.