Gary Levin

USA TODAY

As The Walking Dead ends the first half of its seventh season Sunday, look for more killings at the hands of Negan, the bat-wielding villain played by Jeffrey Dean Morgan. But is the hit series about a zombie apocalypse starting to show signs of its own mortality?

After ratings zoomed for the Oct. 23 season opener, which revealed Negan's victims (including fan favorite Glenn, played by Steven Yeun), they've been on a steady downward spiral in the six episodes that have aired since. A huge 21.5 million viewers — its second-biggest total yet — tuned in for that cliffhanger reveal, with 7-day delayed viewing counted. But in the weeks since, the show has lost more than 5 million viewers, or a quarter of its audience. And Dead is on track to have its lowest-rated season since spring 2013.

‘The Walking Dead’ recap: Enough already

Among young adults, Dead remains the No. 1 show on TV, broadcast or cable. And on its own, the drop is consistent with declining ratings for a few other big hits: Fox's Empire is down 27% this season, and early NFL ratings took a dive. But unlike basic-cable rival FX, which has a growing array of acclaimed and popular series, AMC is uniquely dependent on this one much bigger hit, which airs just 16 episodes a year.

For that reason, the double-digit declines are worrisome for securities analyst Michael Nathanson of MoffettNathanson. With ratings dropping at "an alarming pace, we are becoming increasingly pessimistic about AMC Networks' probability of sustaining the momentum of its biggest show," he wrote in a Dec. 7 report. And the network's newer series, including ratings disappointments such as Feed the Beast, Preacher and Halt and Catch Fire, "have not resonated with viewers or critics the same way that AMC's holy trinity (of) The Walking Dead, Breaking Bad and Mad Men once did."

Why the tumble for Dead, whose spinoff Fear the Walking Dead has fallen even more sharply? Theories abound: Shows naturally decline with age. Some favorite characters are dead. Fans on Twitter don't seem to like Negan much. And the show has tried to top itself with escalating levels of violence, which even Morgan bemoaned in an interview with USA TODAY: "I don’t know if you need to see the closeup gore of it all," he said of his season-opening killing spree. "It’s a lot."

Still, the enduring popularity of a genre show is impressive to Billie Gold of ad firm Dentsu Aegis' Amplifi: "It’s a pretty amazing feat that their ratings have stayed as strong as they have," she says. And while AMC "has a lot invested" in the franchise, with the spinoff and a talk show, it's "considered a holy grail of audience in terms of sheer numbers and social engagement." (Mad Men remained fairly steady during its run, while Breaking Bad's fifth and final season was by far its most popular).

AMC says digital viewership — a combined total of 13 million views this season — is up nearly 50% from last fall, though those views generate less ad revenue.

And there's no sign of the show ending anytime soon. However big the drop, "for a season-seven cable drama to be in its fifth consecutive year at number one is remarkable, and speaks to the passion and dedication of the fan community around this show," says AMC president Charlie Collier, in a statement. "The Walking Dead has never been more alive."