A better man would have said nothing. A smarter man should have made up some lie to cover his arse. Unfortunately, Zacharias Smith wasn't really any of those things. He was just a man who had been stuck in a cell for far too long and never knew when to hold his tongue. "Unless I did do it and wanted you all to think it was out of character for me."
Yeah, that was exactly the sort of thing he should say to an auror regardless of whether he was a friend or on the case. Honestly, that probably wasn't something he should say to anyone. And he really did have it right the first time.