For example, there is a mass shooting or a terrorist attack where lots of people are seriously injured (10+). But nobody dies. Do you think it would be accurate to describe such a crime as massacre? And have there been any such cases in real life?
No, a massacre means a large group of people were there and many of them were killed. Without the killing, it’s definitely not a massacre - you’d need a different word.
Poor Kellyanne Conway got reamed for her use of the term “massacre”. 
You can use it in a joking or metaphorical way like “That actor really massacred his lines”, but if you’re using it in a non-joking way, then many deaths has to be what you mean. And you would definitely never want to use it metaphorically when talking about a situation where you could have meant it literally, because that would be either highly disrespectful or highly stupid or both.
The dictionary says no, society says you can use words however you like.
I mean this literally.
Wasn’t Nixon firing a bunch of people called a massacre once? The memory is fuzzy on this without looking it up.
Correct, literally; the best kind of correct:D
Saturday Night Massacre
[Moderating]
I think this thread is probably a fit for GQ. It certainly isn’t a fit for CS. Moving.
That is technically incorrect.
Arlo Guthrie was involved in a massacre. ISTR he called it a “massacree”, though.
Wiktionary says “massacree” is Ozark dialect for an out-of-control situation. I haven’t been able to find any other source that confirms that, though.
Mass Casualtity Incident is a term used by first-responders. That may be closest what you want. I’m not sure there is a single word to describe such incidents that involve only wounding, no deaths – that seems like a rather unusual occurrence.