So the reason Red John needed to kill partridge is because both PJ and RJ are aware that other people are searching for RJ and very interested in the names on PJ's list. If Partridge is Red John and the list has been stolen, that puts Red John's identity and more importantly, his life in direct danger.

Partridge needed to die first to protect himself from people such as Bob Kirkland, who has already demonstrated that he has no issue with killing....