the cristian who converted

 That was a killer priest who had desired to kill a Muslim all his life. He had no opportunity. That was an easy way to make a dying man a Muslim, who was already dying.

So he, the killer priest, satisfied his ego in an odd way. Whether that man was destined to to heavens or hell is a different matter. That will be decided by Allah, and surely the dying man may have been saved at the last moment by a devil.