OK, actually this quite fits the definition of terrorism by the FBI:
Violent, criminal acts committed by individuals and/or groups to further ideological goals stemming from domestic influences, such as those of a political, religious, social, racial, or environmental nature.
Personally I think that this definition sucks. If I steal an egg to make a statement against capitalism this might as well be considered terrorism. For reference I looked up the German definition of terrorism and I think that this definition is much better(translated with deeply since I'm lazy):
Terrorism is the sustained fight for political goals that are to be achieved with the help of attacks on the life, limb and property of other people, in particular through serious criminal offences such as those listed in Section 129a (1) of the German Criminal Code (StGB) or through other criminal offences that serve to prepare such offences.
The main difference is, that this definition only includes high level crimes and it also states that there has to be an active fight against some form of political system. I wouldn't be to sure if the murder of the UHC CEO would be considered terrorism under the German definition. The reason I'm not so sure is, because I wouldn't count one assassination an active prolonged fight(take this with a grain of salt, I'm not a lawyer or have any serious knowledge on the law).
Don't understand me wrong, I also think that calling this is bullshit, I just want to inform you, that this isn't anything they made out of thin air.