If critical damage ends up killing an enemy, no matter what the weapon, then my GM simply goes "Okay, you kill him, describe how it happens." It generally involves lots of impaling, throat-slitting, or beheading. If there are any additional effects them we apply it afterwards if it fits.
There was one time I set a would-be mugger on fire with a Hellpistol shot, which caused his ammunition to cook off and explode, killing him and several of his buddies. The last survivor (an Ogryn) took (and failed) a Fear test and ran away screaming. I dusted myself off and walked away. It was glorious.
"Oomans are pink an' soft, not tough an' green like da Boyz. Dey'z all da same size too, so dey'z always arguin' about who's in charge, 'cos dere's no way o' tellin' c'ept fer badges an' ooniforms an' fings. When one o' dem wants ta lord it over da uvvers, 'e says 'I'm very speshul so'z you gotta worship me', or 'I know summink wot you lot don't know, so yer better lissen good'. Da funny fing is, 'arf of 'em believe it an' da uvver 'arf don't, so 'e has ta hit 'em all anyway or run fer it. Wot a lot o' mukkin' about if yer asks me. An' while dey'z all arguin' wiv each uvver over who's da boss, da Orks can clobber da lot."