"Those who don't learn from history are doomed to repeat it," They say, and They usually know what They're talking about so it must be true. Watching Inukami 12-13 and then Demonbane 4 right after it perfectly illustrates that The Bad Guys never learn. Dark Helmet may claim "evil will always triumph because good is dumb" but mostly it's the other way around.
Take Inukami first--let's say you're a shinigami, and you have Keita and Youko totally defeated. You can (a) kill them, ensuring your complete victory and putting the writers in panic as they rewrite the next 13 episodes to center on Kaoru and Nadeshiko, or (b) break into maniacal laughter and say you'll spare them for now for no good reason but tomorrow you'll kill them for sure, unless of course they come up with a better strategy next time and kill you instead.
Or take Demonbane--you have the good guys seconds from being messily obliterated, at least except for the surprisingly badass butler ("Winfred" and "total badass" are NOT words I've ever used in the same sentence before), and you're the leader of the Black Lodge. You can (a) do nothing, not even have to lift a finger, and watch your sworn enemy and his budding harem get blasted into their constituent atoms by your minions, or (b) order your lackeys to pull back, because you like living on the edge.
The only explanation I can think of for why if you're the Bad Guy you'd choose (b) in either case is that the writers have threatened you with something heinous in the next episode, like they're going to make sure you spend all 24 minutes alternating between projectile vomiting and explosive diarrhea after eating some bad shrimp, or some other fate sufficiently horrific to make you agree to do something as monumentally stupid as letting the good guys get away.
But it's a formula for a reason, and I have to admit it works on me, since after the shinigami acts thoroughly hateable in Inukami 12, it's hard not to relish him getting what he deserves in the next episode. Especially with the boxing theme I thought I was really watching Rocky VIII there for a minute. And although Adrian never turned into a 25-foot fox with razor-sharp claws and tore the stuffing out of Rocky's opponent, I'm pretty sure some of the later Rocky movies would have been better if she had. Keita has some serious balls to keep trying to cheat on Youko when that's her true form.