In "To Kill a Mockingbird", why would Atticus want Jem to take the blame for the murder of Mr. Ewell or Bob Ewell?
It's a matter of owning up to one's deeds, whether they are justifiable or not. Atticus does not want Jem to grow up with this great cloud over him, so to speak, with people around him staring and talking behind his back. He doesn't want anyone to ever have the chance to say that Jem got away with anything, as a trial will most undoubtedly bring about a verdict of non guilty due to self defense. By owning up to what he has done now, early in life, Jem will save himself years of people wondering what really did happen. Atticus wants everything to be in the open, so that truth can prevail and set his son free.
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.