"It's interesting that the Campbells got such a bad name from that incident when some years before, the MacDonalds slaughtered a group of Mcleods in church.Or is this also a myth?"

There were a great many massacres done by clans including both Campbells and MacDonalds. Glencoe stood out because of the hospitality receivd by the offenders but more so because it wasn't specifically clan warfare (which had all but ended by then)but was in fact a war crime carried out by the state and it went right to the heart of the Scottish govt and to the monarch himself! For instance during the civil war period of the 1640s after the Herschip of Argyll, the MacDonalds and Irish MacDonnells openly boasted about killing 900 Campbells without a sword being lifted in defence. Even allowing for exaggeration it is known that a great many were killed in cold blood. In the same period Montrose let the Irish MacDonnells run riot in Aberdeen massacring the citizens. The Campbells of course were equally ruthless in decimating other clans. I don't think many people in Scotland now seriously worry about it though. It is all distant history.