Should be? Depends on if they'd make for a good story, and if people want to tell it.
I suppose I think of it in terms of "do people want to read it".
And there's the difference between a writer's perspective and a reader's!
But more seriously -- if nobody wants to tell a particular, but readers want to see it, then I think if you do it anyway odds are you wind up with a bad story.
If somebody wants to tell a story, and the readers aren't interested -- well, maybe if the story is compellingly told, they'll change their minds.
But in general, I think the best way to test whether readers want something or not is to offer it to them. Provided it's a story someone wants to tell in the first place.
Some axioms, once undone, can rarely be redone without a total reboot. Killing would seem to be one of them.
If so, then it's already done. I tend to think continuity's a bit more plastic than that -- there are readers who refuse to ignore anything no matter how much they don't like it (indeed, some of these guys seem to insist that the stuff they don't like is so important that they'll cling to an interpretation of something that they hate and talk about it endlessly), but I think most readers are more willing to shovel bad ideas under the rug and agree never to speak of them again.
For instance, if the Metal Men started turning up, and Gold was one of them while that green guy wasn't, and Doc was there, and nobody ever mentioned again that they have human brains, I'm confident that most MM fans would breathe a sigh of relief. There would be some readers who'd keep insisting that this would be violating continuity and it should be explained away somehow and it would color their every reaction to the Metal Men -- but strangely, these would almost all be readers who would rather have Gold in the team and them not have human brains.
Were I writing Superman and thought he should not only never kill but never have killed, I simply wouldn't bring up any past instances where he'd done so, and I'd let readers decide for themselves whether the guy they were reading about in my stories ever had blood on his hands.
But if I had what I thought was a really good idea for a story where Superman killed someone, and I thought it could be done right and was compelling, and DC thought it was good enough that they wanted to do it, then I'd hope enough readers saw it my way to give the story a chance. But in the end, I've got to use my own judgment -- not only is it what I'm equipped with, it's what I get hired for.
kdb