We learn that Tate is going to cover up that Boo killed Ewell. This is ironic because Tate is covering up a murder committed by a reclusive man, who happens to be white. Yet, Heck is responsible to arresting Tom Robinson, who didn't commit a crime but was convicted because he was black.
Now according to Atticus, Heck did everything he could to help Tom - just as Judge Taylor and Atticus did. Ultimately, though, he is willing to protect one of his own, it seems, more than he Tate is willing to help out a black man. After all, couldn't he have hauled Bob Ewell in for obviously beating his daughter? Couldn't he have put two and two togeter (as Atticus does so well at the trial) and realized, because of his deformity, that Tom couldn't have raped and beaten Mayella?
Sheriff Heck Tate is a simple man who shows himself to be determined, pragmatic, and compassionate.
His determination is evident in the fact that he stands up unyieldingly to Atticus, who is an exceptionally strong personality himself. He tells Atticus in no uncertain terms, "I am sheriff of Maycomb County...it's my decision and my responsibility...for once, if you don't see it my way, there's not much you can do about it".
Tate is also pragmatic and compassionate. He knows Maycomb County and sees realistically what will happen if suspicion is directed toward Boo Radley. He knows that the circus atmosphere that would be created would destroy the gentle, reclusive man. He says, "...taking the one man who's done...this town a great service an' draggin' him with his shy ways into the limelight - to me, that's a sin". Out of sympathy and understanding, Heck Tate is willing to bend the law to prevent a greater injustice.