What's New Scooby-Doo?
We're comin' after you,
We're gonna solve that mystery~
Dragon Ball isn't that inconsistent.
The characters are just holding back a tooooooooooon.
The thing about "why don't the heroes just kill the villains?" is that the heroes are walking a fine line; they are, by definition, vigilantes, not police officers, they can fight the criminals, but no more.
It's why Superman, Batman and Spider-man catch Supervillains and leave them for the Police to arrest, they're tackling crimes the police aren't equipped to handle, not playing Judge, Jury and Executioner.