Breitbart was likely assasinated for what someone feared he would release. He was obviously poisened. The fact that the person who performed the autopsy died of poisening shortly afterwards gives strong evidence for this. It is impossible to know that what was released after Breitbart’s death was all that he had or not but it probably was not what they feared that he would reveal.
"First they ignore you, then they laugh at you, then they attack you, then you win!"
"The belief is worthless if the fear of social and physical punishment overrides the belief."