That’s the premise of Showtime’s new dramatic series “Dexter.” Michael C. Hall, formerly of “Six Feet Under,” plays Dexter, a police forensics expert in Miami who, in his spare time, stalks and kills murderers who have somehow escaped justice. If it sounds dark, that’s because it is. It’s also, surprisingly enough, funny, sweet and intriguing, adding it to an ever-growing list of television shows whose central characters traverse moral boundaries that most viewers would never cross.

The TV antihero trend began, to a large degree, with the kingpin of sympathetic bad guys, the emotionally troubled mobster Tony Soprano (chart). Pay-cable channels have long had the latitude to focus on darker characters. But in recent years, basic-cable channels and even the networks have followed suit, from a curmudgeonly surgeon with a decimating wit (“House”), to loose convicts (“Prison Break”) on Fox, to a ruthless cop (“The Shield”) and a narcissistic plastic surgeon (“Nip/Tuck”) on FX. CBS also joined the fray, most notably with “Shark,” featuring James Woods as an acid-tongued attorney.

The exploration of the redeemable villain is hardly new. “Even in the ’70s,” says Robert Thompson, director of Syracuse University’s Center for the Study of Television and Popular Culture, “we had shows like ‘The A-Team,’ ‘The Rockford Files’ and ‘Dallas.’ J. R. Ewing was a terrible person, but an extremely popular character.” Then again, J.R. didn’t methodically murder people, taking a blood sample from each to add to a macabre slide collection. The “Dexter” producers worked with the character created by Jeff Lindsay in his acclaimed novel “Darkly Dreaming Dexter.” But they still had concerns about bringing a morally reprehensible serial killer to viewers’ living rooms. Even the show’s star hesitated. “I took about two weeks to consider whether this was something I wanted to commit to,” Hall says. “I recognized that I was making an open-ended commitment to try to beat life into a character that claimed to be without fundamental human traits, and that was kind of dicey for me.” But the question remains: if Hall himself has to warm to the idea, how difficult is it to get the audience onboard? “It’s really a high-wire act,” says “Dexter” executive producer Sara Colleton. “The irony is, he does live his life by a very strict code that he carefully adheres to, and viewers can respect that.”

If he does live by a code, it’s a drastically different one than most of us follow. In the pilot, we learned that Dexter discovered his compulsion to kill at a young age, and his detective father, Harry (James Remar), trained him to direct his homicidal urges toward other killers who have dodged justice. The two villains Dexter dispatched in the premiere are hardly sympathetic: one is a choir director who mutilates and kills young boys, and the other directs and stars in snuff films that he broadcasts on his Web site. Both men are generally unremorseful; neither is adorable like Dexter.

In a sense, Dexter is making the world a better place one mutilated corpse at a time, and if he has to kill someone, better that it’s the bad guys, right? Questions like these are introduced and left for the audience to digest and untangle. But beyond that challenge, there is a glint of hope that Dexter could redeem himself, a key component of a successful antihero. “There always has to be some humanizing quality for an audience to be able to relate,” says Syracuse’s Thompson. “Even on ‘The Sopranos,’ one of the first things we found out about Tony Soprano is that despite being this ruthless gangster, he has a soft spot for this family of baby ducks that has landed in his pool.”

But when a viewer could just as easily be watching a heroic, likable, traditionally moral character, why invest in the villain with the heart of gold? According to Jenji Kohan, creator of Showtime’s other morally ambiguous series, “Weeds,” about a suburban mother who sells marijuana to pay the mortgage, it’s because heroes show us how we’d like to be, while antiheroes show us how we actually are . “There’s so much social pressure on people to portray this perfect image, and most people are failing because the standards are so high,” says Kohan. “It’s sort of a relief to see someone who isn’t living up to that standard but is doing their best and having moments of triumph. It’s very relatable.”

“Dexter” co-creator John Goldwyn believes conflicted characters create higher-quality television. “Whether it’s ‘The Shield’ or ‘Nip/Tuck,’ I think there’s a greater burden placed upon the craftsmanship,” says Goldwyn. “When you’re in that zone of sympathetic, likable characters in fairly relatable situations, it’s just a much bigger target. We’ve got a much smaller target to hit, and we’ve got to hit it in order for an audience to connect.”

The audience has indeed connected with “Dexter,” judging from its strong premiere ratings and uniformly positive reviews. With 2 million viewers over its eight weekly broadcasts, the show gave Showtime its biggest audience since the debut of Kirstie Alley’s “Fat Actress” last year. Whether that audience will want to continue spending time with a serial killer remains to be seen. CBS’s “Smith” –about violent, high-end thieves with regular suburban families–became the first axed show of the new season, after just three episodes. “Smith” could signal that audiences will walk on the wild side, but they may go only so far, at least on network TV.

The producers of “Dexter” are the first to admit that their show isn’t for everyone. “If you’re not looking to be provoked, this is probably not quite the show for you,” says Hall. Indeed, if audiences don’t stick with “Dexter,” it may not be because he’s so different from them, but too uncomfortably similar. “There’s a part of all of us that feels powerless,” says Colleton. “And deep down, there’s an id that, if left unchecked, could lead people to behave in a way that’s very similar to Dexter.” Not only does “Dexter” penetrate, it twists the knife.