Assuming you don't live somewhere with scorpions...
Sounds like a devil's coachman. It's a beetle, or more precisely one of the largest species in a family of beetles (Staphylinids or rove beetles). If you look closely you'll see it has little shortie wing-cases in the middle, like an earwig does (which of course is unrelated). The wings are folded up tightly under these, and some species fly quite readily (some can't fly though).
Most of the large species in the UK are black, but there are very many smaller ones, many of which are banded black and red. I think they are all predatory, so are a Good Thing in the garden.
They are only pretending to sting, but don't try eating one.