It wasn't easy being in a position of power in 10th Century Europe. But for Italy and the papacy, it was even worse - for a long period, the pope of the Catholic Church was the only person to have authority to crown the next Holy Roman Emperor. Because this was such a powerful position, many people were willing to kill for it.
Sometimes a new pope would pass decrees undoing certain rules of previous popes, or to promote their own personal or political gain. But the case of Pope Formosus was different - he was made to answer for his alleged crimes after his death, by (literally) exhuming his dead body and putting him on trial.
