Terrorism is absolutely a socially constructed idea. The current idea of the charge is based on the moral concept that violence in pursuit of religious or political goals is unacceptable. (You don’t have to accept that but we absolutely teach it in American school so most people just reflexively believe it)

Terrorism as it’s original idea was violence against civilians to terrify them into acting in a certain way. Like the KKK burning a cross in someone’s yard to warn them or a lynching where the entire town attends and takes pictures.

That said a single assassination where the shooter disappears afterwards, leaves no calling cards, makes no announcement, no demands, doesn’t really fit the profile of actual terrorism. They’re charging terrorism because it fits the letter of the law and it’s automatically discrediting to many Americans. Not because it’s actually Terrorism.