As I understand it, the real problem wasn't that the Campbells massacred the MacDonalds, that (in all combinations), was fairly common practice during feud/ clan/ tribal warfare.
The real crime committed by the Campbells was that they carried out this massacre WHILE GUESTS, they broke the rules of hospitality, making them pariahs throughout the highlands when the news spread.
As one of the modern residents of Glencoe was heard to say (on the radio) "I've nothing against the Campbells, I just wouldn't spend a night under the same roof as one"

Walrus