Taking the blame
The longer Nicu was leaving things, the more his mind was going to strange thoughts. The disappointment, the shame he would bring to his family. If he didn't do something then his whole life would be ruined.
When one got to this point, they needed to speak to someone to rationalise their thoughts, a friend to tell them what they were thinking was crazy, but Nicu had no one but himself. He felt like he had to do something, before everything got out of hand.
From the two of the vampire knights that were talking with the inspector earlier on, he spotted one of them speaking to students and decided to approach him. The knight he was approching worked for the twelfth family, the Killton family. These guys always seemed a bit snobbish when someone would speak to them.