That's an interesting question.
To be honest I had never really thought about it before.
It's certainly (now) one of those emotive words designed
to achieve more than simply portraying events.
It definitely conjures up images of innocents being killed
for no good reason.

But it is curious how its use seems to be reserved for certain situations.
The word seems (perhaps oddly) inappropriate for use during wartime.
Not saying the word has never been used in this context.....just that it often
seems to be avoided (perhaps because it is redundant?).
Was the bombing of London a massacre? Was Dresden a massacre? I know....I'm just echoing your own questions here.

For me the word seems to suggest an element of unnecessary and unanticipated
killing.

I think you are right that the word does seem to be getting used as a political
self-serving club for anybody who wants to shcok the world into taking their side.

To your questions I would add:

1) What was it about the events that caused us to call it
"The St. Valentines Day massacre?"

2) Why is it that massacre seems somehow inappropriate for 9/11?
(perhaps because the word is not strong enough?)

3) Is it only a massacre (or not) when viewed in the context of the culture
of those witnessing the event? (one man's massacre is another man's martyrdom?)

4) If you are an animal rights activist...do the daily events at our abbatoirs
satisfy the conditions to be called a massacre.