it's not really about how many people died. it's the context and the motive behind the killings. McVeigh had an agenda against government so he blew up the building. Ted Kacyznski had an agenda against "technology" so he mailed the bombs to technologists. If the person blew up a car with family inside... that's not really terrorism.
This victim is Chairman of the Arkansas state medical board so obviously - his decision may have affected this guy's life.. maybe he rejected the approval that resulted in death of his loved one so this really sounds like a personal vendetta
Murder - you have a personal agenda against certain individual
Terrorism - you have a political/theological agenda against the organization
Mob Hit - well duh you're hired to kill someone
Hate Crime - you have a racist/biased anger toward gender/religion/race/etc.
what else am I missing...