Why is Jem so anxious to tell Atticus what the children know about Boo Radley?

For some reason Jem is under the impression that Boo will be blamed or suspected in the evenings activities. In order to protect Boo, Jem thought it necessary to spill out everything he'd been keeping from Atticus, so that Atticus would understand Boo wasn't dangerous.

Jem seemed to have lost his mind. He began pouring out our secrets right and left in total disregard for my safety if not for his own, omitting nothing, knot-hole, pants and all.

"…Mr. Nathan put cement in that tree, Atticus, an‘ he did it to stop us findin’ things—he’s crazy, I reckon, like they say, but Atticus, I swear to God he ain’t ever harmed us, he ain’t ever hurt us, he coulda cut my throat from ear to ear that night but he tried to mend my pants instead… he ain’t ever hurt us, Atticus—"