Well im on the quest to see who kill the king so i did all i have to do and finally i choose to spare rodrigo's life but now i cant get the promotion of the paladins i dont get the dialog of that is like i'm stuck, what should i do?

thanks, I have to do all again because i want it to be promoted, so did it everything again and kill the bastard :P

I'm interested myself I'm a priest doing the promotion quest but I'ld really rather let him go but I want the promotion. Is there any way around this ?

No.

Actually, it is possible as a priest. To get that, receive your quest from Inquisitor General Commendoni first. After you found all the proofs and if you prefer to let Rodrigo go and let him stay around in the palace alive throughout, continue with the main storyline until you get the members of the Tribunal to agree to arrest Commendoni for being the high priest of the Children of the New Order. The Inquisitor General and your promotion quest giver and examiner will become Cardinal Gustapho Letti instead. You will be able to ask him to administer the quest and exam instead of Commendoni and he will act as if Rodrigo had met an unfortunate demise (Which is not the case as he will still be around in the palace to answer questions). You will be able to become Curial Inquisitor and get the Inquisitorial Investigator this way and Rodrigo will still remain alive in the palace throughout to answer any questions.