I think there's a difference here in what 'officially' constitutes a deathfic as a genre and tastes in reading.
Officially, a deathfic is a fic where the main characters die. Not almost die. Not die and then are miraculously ressurrected. But die. End of story. If the characters are alive at the end of the story, it's not a deathfic.
Minor, recurring or peripheral characters don't usually constitute deathfic either. And certainly very few villains make the genre. Although, there are probably exceptions to the rule there in some fandoms, where the villains have an exceptionally high status within the show.
Now, when it comes to what people like/don't like to read - that's a horse of another colour. And, of course, sometimes it's a matter of differing opinion as to just who is a main character in a show. Sometimes that's not cut and dried. Lois and Clark, of course, in our fandom. But Jimmy? Perry? Cat? Some would say yes. Some would consider them recurring or peripheral. As with most things in fandom, it's never that easy to draw a definitive line.
Often, I think, whether a particular reader is affected by a deathfic depends not on how important or otherwise a character was in the show, but how important that character is to them. How strong an emotional response they have to them.
Bobby Bigmouth, for example, couldn't be called a main character. But if a particular reader really, really liked him on the show, they might consider a story in which he dies horribly a deathfic, where someone else wouldn't.
I've been quite surprised by some of the answers here, so by the looks of things it's probably wise to include a warning if you're going to kill or almost kill off anyone in your story. Just to be on the safe side.
With the proviso, of course, that you don't have a fundamental objection to warnings/spoilers in your story.
LabRat