Have any popes ever been assassinated?

The answer is: Yes.

Several for sure, many more died under shady conditions.

I suspect there are some juicy stories out there, but for now I will leave you with this:

In her book, Saints and Sinners, Eamon Duffy says, "A THIRD* of the popes elected between 872 and 1012 died in suspicious circumstances...

• John VIII (872-82) - bludgeoned to death by his own entourage
  
• Stephen VI (896-7) - strangled
• Leo V (903) - murdered by his successor, Sergius III (904-11)
• John X (914-28) - suffocated
• Stephen VII (939-42) horribly mutilated, a fate shared by the Greek antipope John XVI (997-8), who, unfortunately for him, did not die from the removal of his eyes, nose, lips, tongue, and hands

"Most of these men were maneuvered into power by a succession of powerful families... John X, one of the few popes of this period to make a stand against aristocratic domination, was deposed and then murdered by [those] who had appointed him in the first place" (104).

If I find out any more bloody details, I will be sure to share them.

Until the next question arises,
Your Unemployed Theologian with way too many books

~

Duffy, Eamon. Saints and Sinners: A History of the Popes. New Haven: Yale University Press, 2006.

*formatting added for emphasis