Why is Tate so adamant about protecting Arthur Radley? How does Lee show the reader Tate's intent?

Tate knows that Boo Radley saved Scout and Jem. Should the truth come out, Boo would be paid so much attention that it would destroy him. Heck Tate simply and emphatically states that "Bob fell on his own knife". This,of course, could easily be believed by the public about a guy like Bob Ewell.