In "To Kill a Mockingbird", why would Atticus want Jem to take the blame for the murder of Mr. Ewell or Bob Ewell?

Atticus is most concerned with Jem's personal accountability. He doesn't want his son growing up with some kind of dark past that people are constantly talking about either directly or behind his back.

By using his knowledge of the legal system, he begins to formulate ways that Jem could easily escape charges, like the plea of self-defense. However, Sheriff Heck Tate assures Atticus that Bob Ewell "fell on his knife," despite evidence that Boo Radley stabbed him in defense of the Finch children.