In To Kill a Mockingbird, what is the difference between the letter of the law and the spirit of the law?

A great example of the difference between the letter of the law and the spirit of the law can be seen in chapter 30 after Boo Radley saves the Finch children from Bob Ewell's attack on their lives. Atticus is a proponent of upholding the law even if it means that his son has to take responsibility for killing Bob Ewell in self-defense. Heck Tate tells him that it wasn't Jem who killed Ewell, but Boo Radley. As sheriff, Tate represents law enforcement, so what he determines in the case can be analyzed by the state. If anyone is to be held responsible for Ewell's death, which would be based on self-defense, it would all be determined from Tate's report. Tate tells Atticus the following about his decision:

"To my way of thinkin', Mr. Finch, taking the one man who's done you and this town a great service an' draggin' him with his shy ways into the limelight--to me, that's a sin. It's a sin and I'm not about to have it on my head. If it was any other man it'd be different. But not this man, Mr. Finch" (276).

Heck Tate is basically saying that the spirit of the law overrides the letter of the law when it comes to Boo Radley. He's making the point that it is better to honor Boo for his good deed by allowing him to keep his right to privacy over reporting everything to the police, town, and state. Tate also seems to allude to the title and theme of the book as stated by Atticus in chapter 10:

"Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird" (90).

This seems to be the spiritual law discussed in the book. It's a sin to hurt people who are less-fortunate, or who have fewer privileges or opportunities based on race or social ability. Atticus realizes that Tate's decision is better for Boo Radley, who is like an innocent mockingbird, and tells Scout to support it as well.