we learn that mr. wewll had actually threatened to kill atticus. how does atticus take the threat?

Atticus feels that Bob Ewell's threats are all bark and no bite. Atticus knows that Bob has been humiliated and that spitting at him is the only way that he knows to save face.

Here's the quote should you require it,

"Jem, see if you can stand in Bob Ewell's shoes a minute. I destroyed his last shred of credibility at that trial, if he had any to begin with. The man had to have some kind of comeback, his kind always does. So if spitting in my face and threatening me saved Mayella Ewell one extra beating, that's something I'll gladly take. He had to take it out on somebody and I'd rather it be me than that houseful of children out there. You understand?"