When Severus discovered that his son Killian was responsible for the deaths of the five vampires, he headed straight to his son's house. Because Lazarus was apprehensive about what may happen, he instantly followed his father in order to avoid any unpleasantness that might arise later when he did not see it coming as soon as he could.
Lazarus could not really drive his car effectively because he was frightened about his brother, who would undoubtedly hear their father say harsh things again.So he contacted Killian a few times, but perhaps he was not holding his phone as steadily as normal. That's why he couldn't respond, adding to Lazarus' uneasiness at the time.
