The MacDonald bit as is said could well be a later addition but not necessarily because of Glencoe. The two clans were committing atrocities on each other through the 1600s and before. In fact Glencoe gets remembered but it wasn't by any measure the worst atrocity - and wasn't even really a clan action at all. It was planned and ordered by the Scottish gvt in Edinburgh with the agreement of King William in London. The Argyll Regiment may have been used by the gvt and the commander was a Campbell but only about 10% or so of the troops were.

The two clans did much butchering of each other and writing about it though. During the civil war period in the 1640s and the devastation of Argyll the MacDonald bards boasted about killing 900 Campbells without a sword being lifted in defense. Maybe an exaggeration as bards were apt to do but it suggests many non combatants were slain. Here is a translation from the Gaelic by Florence MacLean of Coll who was a Campbell by birth but wrote about wishing to fight her husband's clan and the MacDonalds.

"Were I at Inverlochy,
with a two edged sword in my hand,
all the strength and skill I could desire,
I would draw blood there, and I would tear asunder
the MacLeans and Clan Donald.
The Irish would be without life,
and I would bring the Campbells back to life"

There were literally many hundreds, probably thousands, of Campbells who died in battle or were massacred prior to Glencoe. As there were MacDonalds on the other side. From the small lyric in Bonnie George Campbell which doesn't really give any detail it is surely impossible to pin the story down to a certain individual. Unless there is some actual evidence for it other than someone suggested it might be so and so!!