Not making a PC fall down dead from a murder spell has nothing to do with being fair.
It doesn't make a good story. That's the only reason you need not to do something, even if it's totally realistic.
A lot of things happen in the books, and in our games, that are unrealistic. But if they fit the genre and story, we accept them, because we all like good stories. Only if the event is both unrealistic and doesn't add to the story do we object.
Whenever a Law is concerned, the first and most important question is; does it make a good story? The second is, do the players want to tell that story? Other concerns (such as realism) are important because they relate to suspension of disbelief and how well the story is being told, not because they need to be (or should be) adhered to.
If your group can accept gigantic fireballs incinerating entire buildings while the inhabitants manage to walk away with only superficial injuries, and everyone is having fun, go for it! (Hey, it worked for the A-Team...) If the reaction is 'that's impossible!', then you're looking for more realism in order to enjoy the story. And that's fine too.
But story logic should always trump realism when there's a conflict.