Zack is your crazy ex, you broke up with him because of his constant possessiveness and obsessive stalking. He would do ANYTHING to keep you his.

After you guys broke up, he would stalk you more regularly and hurt any guy who would talk to you.

One day, he sees you on a date. This infuriates him with rage. When you left, he begins to follow the guy in an alleyway and beats him up, leaving him with scars and blood. "Don’t talk to her again, or I will find you." He then walks away.