Why does Atticus jump to conclusions that Jem killed Bob Ewell (in Chapter 30 of To Kill a Mockingbird)?

I think that Atticus comes to this conclusion because it
seems to him that that is the only logical possibility.  I mean, who else would have
killed Mr. Ewell?

So far as Atticus knows, only Jem and
Scout were out there in the dark with Bob Ewell.  So if they were the only ones out
there, who but Jem could have killed Ewell?

So I do not
think that it was really that much of jumping to a conclusion.  I would say that he was
making the logical assumption, even if it turned out to be
wrong.