Sadly, these harmless and gentle folk were target of scaremongering and incitement. So much so villagers from surrounding areas, who'd lived with the strange tales around the isle for hundreds of years prior to the massacre, were fed lies to harness their fear to such a degree they were the ones responsible for the grisly murders that night.
The perpetrators of the incitement - the local clergy of the time - had everything to gain by ridding the land of these 'satan worshippers'. The harmless folk who were on the receiving end of the churches wrath, were practitioners of the ancient arts of the land that had been so for hundreds; if not thousands of years prior to the church's pillage and destruction of the lands of Albion.
Ynys Wydryn was a special place, once, the rocky outcrop we know as The Tor in modern times, is a hollow hill with reputed wonders below ground. After the massacre of Ynys Wydryn, the church took possession of The Isle and reputedly destroyed all the artifacts and wonders that were kept for safe keeping on The Isle. They plundered the technology that ancient beings has bestowed on the custodians of Avalon. Then to rub salt in the wounds, they built St Micheal's Tower on the tor as a arrogant symbol of their seizure of such a prized pagan site.
However, the magic and the power of the isle could not be destroyed, despite the church's best efforts. Its secrets may well be long destroyed, but its truths are written into oral legend and myth by those who are descendants of those massacred that night.