It was not Lucifer's ideal plan to come to the castle all of a sudden. There had been a day where he thought he would come and he was sure the day he was supposed to meet Ian was not today but then the situation called it. While seeing the sight from Beelzebub of Ian being harmed, Lucifer knew that it was fake and he could have ignored it but there lies the slight possibility that it was not an act and having lost his sister, he could not afford losing his nephew due to his small careless mistake.
He had taken the job of protecting Ian in his late sister's stead after seeing her ashes and being a person of his words, he would not allow any harm come over Ian who is the last proof of his sister's being.