That's an interesting question. I think it perhaps comes down to the question of whether terrorism is purely defined as violence in pursuit of a political, religious or social ideology, or whether it has to be organised violence in pursuit of... If the former, both, if the latter, it becomes more nebulous.
Let's take the IRA as an example of typical western European urban terrorism towards the end of the last century. They (if I understand the situation correctly) had a command structure a bit like a regular army, with attacks being planned by higher-ranking strategic specialists, materiel supplied by "quartermasters", attacks carried out by low-ranking foot soldiers (generally not suicide attacks).
Elliott Rogers, on the other hand, while influenced by online misogynists, wasn't actively recruited by an anti-women group with any sort of structure.
As far as I can tell from the news reports, the current spate of attacks occupy a middle ground which may be a new phenomenon. The attackers don't merely interact with other people who share their ideology on line, there seems to be a sense in which ISIL are engaging in on-line grooming - seeking out people with feelings of alienation or mental health problems which make them vulnerable, winding them up and letting them go. It's not planned meticulously like an IRA "spectacular", it's more of a "wind them up and let them go and see what happens" strategy. However, the further arrests in Nice suggest that there may be a bit more organisation to it than just internet grooming - there is some supplying of guns, explosives, etc. going on.
But I don't think it's helpful to have a sound-bite approach to explanation which says "they left a video on their phone proclaiming loyalty to ISIL" = terrorist, whereas Jo Cox's murderer, Anders Brehvik, Elliot Rogers = not terrorist because no underlying organisation (or, the cynic in me thinks, "not far enough up our list of threats that matter to the establishment politically speaking").
I think it is interesting to note that most of the perpetrators are young men.