The bad guy is never, ever dead. Not until you see him crushed into a trillion particles of dust and scattered to the four winds is he definitely gone forever and frequently not even then.
A rule which can be applied in many ways and many places.
Horror movies are the most obvious applications of this rule. The bad guys in horror movies are frequently supernaturally motivated or something, making them essentially immortal, able to survive falls and take hits that would instantly kill any ordinary human being. Even come the end of the movie in which the bad guy is apparently gone, there is usually a sequel, in which some crazy chain of events or something conspires to wake the bad guy up again.
Dragonball Z is similar. And people in that show are dumb. Goku's just landed his biggest, strongest attack directly on his enemy's head, and everybody starts celebrating before the dust even clears. Here's a hint: BAD GUYS ARE ESSENTIALLY IMMUNE TO DUST. Most likely your attack didn't even scratch him. Until you see the bad guy annihilated, decapitated or whatever with your own eyes, it's not over. Ever. And with the Dragonballs themselves around, capable of bringing people back from the dead, almost nobody stays dead anyway.
It's used elsewhere too. James Bond's long-running nemesis Ernst Stavro Blofeld is a good example: if memory serves, Bond "killed" him a bundle of times over before finishing him in the opening scene of For Your Eyes Only (do correct me if this is wrong). Star Wars too: Vader dead? All you saw is his TIE Fighter spin out of control. He'll be back.
The Immortality Principle is often used to a lesser extent in bog-standard action movies. The moral of these stories is usually that flinging the bad guy to his apparent doom is rarely if ever fatal. This allows the good guy to apparently kill the bad guy, rescue the girl, have a romantic chat/snog, possibly hurriedly vacate the building before it explodes, then, for a final sting before the credits, be ambushed by the bad guy, who actually survived and escaped. Usually in these cases the bad guy is definitely dead at the end, though, so as to provide closure... unless of course they're setting up for a sequel.
A corollary of the Immortality Principle is that if there is a recognisable bad guy in a movie, then there is always room for a sequel. It doesn't always get made, but if the original was profitable then it's a good bet.