They were killed because their ideas and the fact that people were following them and listening to them presented a threat to the status quo and the interests of whoever it is that really runs things.

Who is that enigmatic someone?
I dont know, but i have seen read and heard enough to convince me that most world leaders are basicaly puppets.

Think about the ideas that Lennon was talking about before he was asassinated. True world unity, real kinship between all the peoples of the world. In a world like that there is no need for leaders or governments. If you are in a position of political power then those ideas would be pretty threatening.

Oh and just in case it comes up "Imagine" is not about communism. The idea behind that song trancends any form of government.