The MacDonalds did indeed attack and kill MacLeods while they were in church. It was in retaliation for the Eigg Cave Massacre in which about 400 MacDonald men women and children were killed by the MacLeods. Scotland's history runs dark with bloody fueds resulting in events much worse than the Glencoe Massacre. What sets that one aside was the treachery of the enemy posing as friends. There was an unwritten code that was violated and that was considered a greater crime than the killing.
Due to the Highland Clearances many descendants of these people landed on Cape Breton Island, and there is also a place named Glencoe here where both Cambell and MacDonald have live peacefully side by side.
                     Slainte,
                         Sandy