Saying an idea is dead is a metaphor, so it doesn’t have a concrete meaning.
Some people say something is “dead” when the people they hear talking most often don’t talk about it every day.
Some people might mean when they haven’t heard it in a year, or a decade.
Others might mean when it’s not on the Internet.
Or when it doesn’t seem to be being thought about currently at all by anyone.
But it’s still just an idea, that never actually lived or existed, except as an idea, which I would say ultimately is something timeless and that doesn’t require anyone to think of it.
1 + 1 = 2 and 1 + 1 = 3 I would say refer to ideas that were bound to be thought of as soon as there were thinking creatures. And if Huffman hadn’t thought of Huffman encryption algorithms, someone else would have very soon after, because it’s a natural logical thing to do. Patents are an economic argument against the truth that new (to humans) ideas are not really created out of nothing by people, but found by trying different combinations of other basic ideas which were originally just based on logic.
So the definition of life or killing for an idea is arbitrary, and needs to be defined to have a concise discussion about it.
For myself, the ideas I would like to “kill” tend to be those I disagree with, dislike, and/or feel are leading to needless suffering or waste of time and/or resources.
Usually I think this means coming up with alternative better ideas, and/or pointing out the negative things about the target ideas.
However, few if any ideas, especially in popular consciousness, stand alone. Ideas get repeated in conversations with others, and in recorded media, and are associated with many other ideas and emotions and stories, which in turn are associated with others. Often an idea will be supported by many other ideas and emotions with which it resonates.
The most effective ways to unravel and change conversations tend to come from gaining a larger sense of the whole swirl of ideas and emotions, as they exist in various audiences, and then creating a counter-swirl – an alternative resonating patterns which can do some jujutsu to use the energy of the original swirl and turn it into a new and more powerful swirl going in a better direction, and that avoids getting mired in vortexes of stupidity or other negative patterns.
A simple example was the sudden shift from “disco fever” in the 1970s to “disco sucks”, basically a graffiti message that caught a dance fashion craze and had it go from trendy to embarrassed almost overnight.
Another example might be political candidates who stand a chance but then get dismissed as soon as some negative idea about them gets enough attention, even when it’s really stupid and unfair.
Then of course there are industrial idea-blasting campaigns that sidetrack conversations that they think will cost them money, by buying and suppressing certain researchers and journalists to get them to spread doubt and distracting conversations, and who also pay people to even go out on the Internet and argue their way in online conversations.