Why was Paul Revere's rendering of the Boston Massacre considered propaganda, and not objective reporting?

That is because he was biased. He depicted it as if it was an execution with British soldiers standing in line and shooting at innocent people, while in reality what happened was that a soldier was getting harassed by a mob and then more soldiers jumped in to defend him and there was something like a riot. Fire opened on the people in defense and it was chaotic, it wasn't like standing for execution.