He had a history of heart problems, and despite all the conspiracy theories, likely succumbed to a heart attack. Breitbart’s lifestyle (he only got 4 or less hours of sleep a day - the man was driven) was a major contributing factor.
Do you have any valid references for that assertion?
A cursory search on Google shows only current posts with suppositions and assertions but without references to historical documentation.
Either there are no earlier references or no one has gotten around to planting them yet.