The red colour of the water comes from the sandstone of the streambed, and the sources say plainly it is not supernatural, though it may be the reason the dark stories grew. The glen was used as the setting for 'St Ninian's Spring' in the television series Outlander in 2014, and now gets tens of thousands of visitors a year, according to Wikipedia's estimate.
Wikipedia's article does not itself record the Devil legend. Travel and curiosity sites do, and the druid and witch-execution claims are not supported by any record I found.
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