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As sheriff of Maycomb County, Heck Tate's official role is maintaining law and order. Through most of the book, however, he seems about as powerful as Ranger Smith faced with Yogi Bear and a missing picnic basket. Check out the list of failures:
But in the end, Tate faces off against Atticus, and wins:
"I never heard tell that it's against the law for a citizen to do his utmost to prevent a crime from being committed, which is exactly what he did, but maybe you'll say it's my duty to tell the town all about it and not hush it up. Know what'd happen then? All the ladies in Maycomb includin' my wife'd be knocking on his door bringing angel food cakes. To my way of thinkin', Mr. Finch, taking the one man who's done you and this town a great service an' draggin' him with his shy ways into the limelight—to me, that's a sin. It's a sin and I'm not about to have it on my head. If it was any other man, it'd be different. But not this man, Mr. Finch. […]
"I may not be much, Mr. Finch, but I'm still sheriff of Maycomb County and Bob Ewell fell on his knife. Good night, sir." (30.62-63)
Tate's decision may go against the letter of the law, but it follows a different, higher (to him) law. Depending on whether or not we agree with his decision, Tate is either doing the right thing or weakening the rule of law (or both). He acts on moral grounds, but in doing so he goes against Atticus's ideal of equality under the law.
Maybe one message of Heck Tate's character, and Link Deas's as well, is that until that ideal becomes more of a reality, people should do what they can to bring fairness at least a little bit closer, even when the law's not on their side. The problem with that message? The same principle could be used to justify the lynch mob's attempt to enact vigilante justice on Tom.