Just go to the trap door behind the Servants Guild and go into the room with all the Paladins, kill one and then a guard will notice you and knock you out. You will wake up in Ardougne Castle. Try going now to a small house near the marketplace and talk to Paladins, she will follow you this time to the Paladin hideout but you will need some Paladin armour just pickpocket some Paladin cult members. Congratulations! Quest complete!