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.

There were a number of inaccuracies in the image that were made to show the British soldiers as being more brutal and calculating than the situation actually was. The actual massacre was more chaotic and more disorganized than an actual battlefield scene. There were fewer soldiers from the British army than were depicted in the scene, as well.