Is allowing a player to be LE and Worship Asmodeus going to break this AP. He understands that he can't break the party by being evil but i'm just wondering if it's going to be to much if he follows Asmodues's rules?
Surprisingly simple, actually. There are a ton of places in Serpent's Skull where I figured "if the group was evil, this would be so much easier." Make sure he's the sort of Lawful Evil that:
A. Won't sabotage (includes betrayal, but also other things) the party.
B. Will oppose the serpentfolk and Ydersius.
C. Will oppose the vast number of chaotic evil deities worshiped in Saventh-Yhi.
Also make sure your group has no paladins or particularly noble sorts. A lot of Serpent's Skull is about making decisions, some more "good" than others. A lot of conflicting morals between people will cause a ton of problems down the line.
This is pretty much one of the better AP's to allow evil or greedy neutral characters.
Having an evil party member is fine as long as he sticks to the points Shimnimnim made.
Having an evil/neutral party might even open up some of the normally avoided factions like the red mantis or aspis consortium.
Or at least get you away from the pathfinders which seem to be pretty much the go to faction for all the players I have spoken to.
Oh, that's actually another thing to keep in mind:
Having an evil party member makes alliance with several tribes in City of Seven Spears easier. It also allows a person to choose to travel with the Aspis Consortium or Red Mantis, usually sort of "no no" choices for their evil actions.
Keep in mind though that it's a natural GM decision to rule the Pathfinders as sort of well intentioned but ineffective individuals. In fact, I wrote the Pathfinders in my campaign as being just as effective as the others, but my party still feels they're not. Keep in mind that you'll need to be careful about keeping the side of "good" powerful enough to be threatening, else you'll lose a good part of the game.
I definitely get the feeling that the pirates and the Pathfinders are meant to be the PC teams, and the Aspis Consortium and Red Mantis were written as the villain teams. Therefore, it might actually be safer to use one of these guys as the "against the PCs" anyway, using the opposite of the one they join.