A random civilian getting shot in the eyes by a neighbor who doesn't like his political views is not being Breitbarted. A man with damning evidence and testimony against an elected official who suddenly shows up in a dumpster decapitated and it's ruled a "suicide" by the coroner, however, definitely qualifies. Most victims, however, die of heart attacks - which can be brought about by hard-to-trace poisons. The difficulty is proving it.
In summary, you need:
1) Reasonable grounds for suspicion of foul play.
2) A clear government motive for assassination.
3) Convenient circumstances.
4) Someone in government as a prime suspect. Not the Mafia, unless the two are working in cohorts.
5) Impeccable timing.
Absurd coroner rulings are optional. This usually applies to conservatives who die suspiciously and to liberals who are suspected, but any combination of ideologies is possible for a victim-assassin relationship, provided the other above-stated qualifiers are present.
Named after Andrew Breitbart, the timing of whose death raised lots of suspicions.