An entire book was written about JPI's suspicious death by a respected investigative journalist named David Yallop called "In God's Name." It is very compelling. Indeed, JPI was about to open the lid on corruption in the Vatican bank. The book contends that high-ranking conservative prelates in the Vatican who were suddenly out of favor in the new, more liberal regime strongly hinted to their mobster relatives that they were about to be exposed. That's all it took for the mob to poison him, as they had Paul VI, and the rest is history. The conservatives made sure that the ambitious young Karol Wojtyla, who had made it pretty far the previous month, was there to lead the conservative restoration to the papal throne. Naturally they succeeded. Whenever conservatives lose fair and square they always resort to treachery in order to win the next time, e.g. gerrymandering, Neil Gorsuch, etc. 

Oh, one more thing about JPI - he wanted to allow same-sex partners to adopt children in Italy.