I've always wondered why Atticus jumped to the conclusion that his son was capable of killing Bob Ewell. I feel that Atticus was so distraught about Jem's injury and his children's brush with death that he was simply not thinking clearly. It is not true that he was unaware of a possible fourth party lurking in the darkness that night: Boo Radley had been hiding in the shadows of Jem's bedroom during Sheriff Tate's questioning of Scout, and Atticus knew he was there. He must have also known that Boo had something to do with bringing his children back to the Finch home. Why Atticus did not consider Boo as the possible killer of Ewell is unexplainable. Again, Atticus was not the clear thinking man that he was in the courtroom during the Tom Robinson trial on this night or he clearly would have realized what Sheriff Tate had already figured out.

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.