In chapter 10 of "To Kill a Mockingbird," why does Heck Tate not want to shoot Tim Johnson?

On page 104 Lee writes, "Take him Mr. Finch."  Mr. Tate handed the rifle to Atticus; Jem and I nearly fainted,

"Don't waste time, Heck," said Atticus. "Go on."

"Mr. Finch, this is a one-shot job."

Atticus shook his head vehemently:"Don't just stand there, Heck! He won't wait all day for you -"

"For God's sake, Mr. Finch, look where he is! Miss and you'll go straight into the Radley house!  I can't shoot that well and you know it!"

"I haven't shot a gun in thirty years-"

Mr. Tate almost threw the rifle at Atticus.  "I'd feel mighty comfortable if you did now," he said.