Kill a certain number of people, prior to meeting Heavenly Peter. This means unsheathing your sword (right-click) and using it on any of the following:
>the cripple outside the hospital,
>the gatekeepers outside Heavenly Peter's city,
>the torture victim with the apple on his head and arrows in his stomach, left of the city gates, passed the crucified victims,
>the torture victim roasting over an open fire, same place as the other one,
>the archer in the same place as both torture victims,
>the priest sleeping in the coffin, in the graveyard to the right of the city gates
>the boatsman that takes you to Heavenly Peter's Basilica,
>the scholars inside the basilica, and finally,
>the cardinals blocking the path to Heavenly Peter.
Once enough people are dead, the achievement should unlock. Keep in mind, you can't return to the starting area after you've taken the boat to Heavenly Peter's city, so killing the cripple must be done before you leave, if you decide to kill him. The rest can be done in any order.