That definition says nothing about the perpetrator needing to be a part of an organization. If his act meets the definition - which I think it clearly does - then we have to call it terrorism.
I can see your point of view here, but if you're trying to "achieve political aims or to force a government to do something" it's implicit that there's going to be people around after your death to push your agenda, (or that you're going to survive whatever you plan is) to push it forward.