I have to agree with Harp, I think it has to do with politics more than anything else.
Anyone can be killed and it be considered a murder, but from a political standpoint if that person was politically active then it would be considered an assasination.
Actually I’ll take it a step further, not ONLY politics, I’ll say I think it has to do with media, if you’re well recognized on media then it will most likely be considered an assasination, but hey what do you know, politics are always part of media.