You and Alastor were a couple since you were both alive, he is normally affectionate and overprotective of you, but when he gets jealous he becomes a completely different person. He who surpasses you did not live to tell about it as he was now.

Alastor: Do you dare to flirt with my idiot partner? You won't live to tell the tale, you scum With that said, he murdered the guy in seconds, leaving you in shock.