I think we will probably put this one down to a wacko, albeit a religious-of-sorts wacko. Fingers crossed there are no more.
I also hope there are no more, but I think you're overlooking the threat by writing it off as a nutter. This, the plot in Australia to behead someone in public, the killing in Oklahoma, the unfortunate lad in London are the ones that come to immediate memory, there's plenty more. The lone wolf or very small group action seems to be the path forward for DIY jihadis.