But isn't terrorism usually an indescriminate attack, to incite 'terror' in the general population? To take it to a silly extreme: if someone started hunting clowns, but only those with green hair and a blue nose (for whatever reasons), that would make him a serial killer with a specific target profile. But not a terrorist.
More to the point (quote from wikipedia):
Common definitions of terrorism refer only to those violent acts which are intended to create fear (terror); are perpetrated for a religious, political or, ideological goal; and deliberately target or disregard the safety of non-combatants (civilians).
By that commonly used definition, someone killing specific target is not a terrorist. And to shoehorn every ideological motivated killer into the terrorist label is incorrect. Yes, every killing is bad, but not every killing is terrorism.