> Murder is when you kill someone on purpose, when you had the option of not.
Indeed, I think that your original reply would have been more useful and less apparently combative if you had
1. replied with this definition, 2. showed why this commonsense definition that does not definitionally involve morality is useful, and gave examples of it, 3. finally showed marginal cases where murder is not a moral bad according to certain ethical frameworks, 4. acknowledge that it's OK to redefine the word so that it is definitionally morally bad, but since it differs from a commonsense notion, it needs to be signposted,
but I think you had a bit to figure out yourself.
I'll have to study my original comment to see why it appears combative: thanks for pointing that out.
RE 2: I just looked through a few country's English-language laws and cribbed (what I saw as) the consensus definition of "murder". I'm not sure the other definitions I gave actually correspond to any real-world legal system, but many of them have something similar.
RE 4: Defining the word as "definitionally morally bad" means you have to make a moral assessment before using the word – but that takes most people way longer than the language processing of two syllables, so it leaves you open to equivocating rhetoric. Appeals to "common sense" (which, in my experience, is far from common) aren't why I object to that definition (except as signposted technical jargon, narrowly-scoped to a particular context).
The post I originally replied to was equivocating in this way (probably unintentionally), hence the strong objection. Though in my experience, explanations are better than intensifiers: that's something I should look out for in my editing passes in future.