Sheriff Heck Tate's response to Atticus concerning the death of Bob Ewell has its roots in The Bible. In the Book of Luke 9:60, when Jesus encountered a man reluctant to join him because he first had to tend to the funeral of his father,
Jesus said unto him, Let the dead bury their dead: but go thou and preach the kingdom of God.
Jesus was trying to convince His potential follower to let those who are spiritually dead tend to the deceased, while the living should follow Him and preach the word of God. Sheriff Tate takes the Biblical quote somewhat out of context, but its meaning is still clear: Since Bob Ewell, the man responsible for the death of Tom Robinson, is now dead, there is no reason to bring Boo Radley into the picture and blame him for Bob's death (though it was self-defense). Boo is still alive--physically and spiritually--and
"... taking the one man whose done you and this town a great service an draggin' him into the limelight--to me, that's a sin.
Boo is symbolic of a Christ-like character at the end--an innocent man who is willing to sacrifice himself for his young followers, Jem and Scout--and Tate believes charging Boo with Bob's murder would be akin to the crucifixion of Jesus.