I think if Michael Zehaf-Bibeau is considered a terrorist, then the guy that killed Jo Cox should be considered a terrorist.
But does the attack actually classify as terrorism? The main problem is that there is no consistent definition of terrorism.
Terrorism usually involves violent acts to achieve a political purpose, but so many things would satisfy these two requirements. For example, a war due to a trade embargo or some petty squabble between monarchs would be considered terrorism (so a lot of the European wars would be considered terrorism). How about someone assassinating Hitler during the 2nd world war? Would that count as terrorism?
I've often seen 2 more conditions put on terrorism:
1. The targets must be civilians. So attacking enemy soldiers in war does not classify as terrorism.
2. The attacks must be performed by non-state actors.
For 1, do MPs classify as regular civilians or not? In the case of Zehaf-Bibeau, he targeted a soldier and MPs (who were arguably responsible for Canada being at war with ISIS, which was a group Zehaf-Bibeau identified with). Similarly, for this attack, Jo Cox was an MP.
For 2, does ISIS could as a non-state actor? Given that ISIS controls a large amount of territory, enacts and enforces laws, collects taxes, has a standing army, etc. they satisfy the definition of a state. If one accepts condition 2, then does this mean that ISIS is not a terrorist group? If ISIS is a terrorist group, then for consistency condition 2 should not be part of a correct definition of terrorism. However, this means that the fact that the guy that killed Jo Cox was not operating as part of a non-state actor should not be used to argue that the guy classifies as a terrorist.