Why aren’t you watching “Dexter” yet? Even if you don’t have Showtime, go rent the previous seasons to see what you’ re missing. Thankfully, there are plenty of fans out there keeping the serial killer drama hotter than ever – and it only helps that John Lithgow is starring as this season’s killer. A week before its season finale, “Dexter” has already posted ratings highs for Showtime. Sunday’s episode of “Dexter” averaged more than 2.1 million viewers at 9 p.m., repping the most-viewed single telecast for Showtime in more than 10 years. That also made Sunday’s seg the most-watched episode of “Dexter,” now in its fourth season, EVER. Showtime hadn’t attracted a larger audience for a single telecast since Oct. 1999, when it ran the Tyson vs. Norris fight. Next week’s “Dexter” finale will feature a final showdown between Michael C. Hall’s lead character and rival serial killer Trinity (John Lithgow).