(or "The Perfect Villain")
Half of the fun of reading Marvel's "Essentials" series is getting the big picture as to comics trends in different eras. 'Werewolf By Night' was just one of many horror comics that Marvel put out in that era, as the rules for publishing horror comics had relaxed a little by the 1970s. It was interesting, to see this sudden secret part of the Marvel Universe pop up where werewolves, vampires, zombies (or, in the parlance of the time, "zuvembies"--not all of the rules had been relaxed), and even Frankenstein's Monster could rub shoulders with Spider-Man, Iron Man, and the Fantastic Four. A world where the good guys didn't always win, and even when they did, the monsters were still out there, waiting for another chance.
Which is, really, what Jack Russell's story is all about in 'Werewolf By Night'. He's a Nice Young Man (with a bit of a sarcastic streak, but he's had a tough life) who's inherited a family curse to get all mean and hairy on the three nights of the full moon (and anyone who's read the series can never forget those immortal captions that introduced the werewolf sequences. "First Night:" And you knew things were about to get interesting.) Naturally, he's not excited about the idea of turning into a monster, and is constantly trying to find ways to cure or contain his curse.
To some extent, this is just another "false status quo" series. Jack is always trying to find a cure, but the second he does, the series ends, so that's a dead end plotline (although they did some clever things over the course of the series with his gaining more control over his wolfish side.) But the werewolf isn't just a mindless ravening monster, he's a character in his own right. He's as much a protagonist as an antagonist, as much a part of the series as Jack is himself. That, in turn, opens up several options for the enterprising writer. Obviously, Jack Russell isn't the first guy to have a split personality. It's a favorite device of cult fiction, from the Hulk to Mister Hyde to...well, Mister Hyde, in the excellent BBC series 'Jekyll'. But it's worth looking at for what it gives to the writer's storytelling engine.
For starters, it gives the writer a villain that always has a legitimate reason for turning up. One of the toughest parts of any open-ended series is finding new ways to create conflict, getting good antagonists to show up and stick around. With a dual-personality engine, the two personas can always find different ways to conflict with each other, because neither one of them likes sharing the body. The simple logistics of living a life when you're not always in control of your own actions can generate virtually endless stories, as Jack Russell always wakes up three mornings a month trying to figure out where he's been and what he's been up to.
In addition, it also means you can add more supporting cast members and villains, and play them off against each other. You can have a villain who wants the werewolf as a pet or an ally, but doesn't care about Jack. You can add new relationships, complicate existing ones, bring in characters who only know one side of the duo, and still be able to tell standard "superhero saves the day" storylines. (It's always an important element of stories like this that the "evil" side of the hero is just noble enough, deep down, that they're disgusted by villains worse than they are, and find reasons to save the day after all. They're antagonists, but they're leashed by their other half just like the hero is.)
Really, the only frustrating restriction in 'Werewolf By Night' (apart from the fact that it puts the 'Moonlighting' theme in my head every time I read the title) is the fact that they chose to stick to the purely lunar changes. It's a restriction that makes it difficult to tell stories the other 27 days of the month--although they do find ways around even that over the course of the series. But being able to play your two protagonists against each other, with neither one able to score a decisive victory, makes the series perfect for an open-ended storytelling engine. Jack Russell might be a monster, but he's in good company.