An interesting development

So today in my groups weekly session a new twist came to upon my players. One of the party members died while attacking a level 20 cleric (he's a level 3) in the moment of his death he called out to a powerful demon that another player worships. I allowed it to happen and had the demon offer resurrection for fealty. Much to my surprise he accepted the deal. Now we have two chaotically evil demon worshiping and a poor neutral player stuck with them out of fear for his own life. the other half of the party are good natured people and it seems as if pvp is very quickly going to occur and I dont know how to handle that, ive never run a campaign in which the players turn on one another so I'm in need of some advice, please fellow DMs help me through this struggle!