Maybe Matthews will try and confront or threaten Dex and he kills him. I'm still suspicious of him as a character, so duplicitous. Although maybe he's just there to symbolism the corruption of the police system or something, the very inspiration for Dexter to take the law into his hands. How many times has he forced a law officer to let a guilty party go free? Unless he somehow knows that by doing so... it leads them to Dexter's table. Does he know Dexter is a killer? I honestly can't recall.
He has had me very suspicious for a while now. I wouldn't be surprise if we learned that he knew about Dexter all along.