Who would consider me a whore after dating my ex's best friend?

I was dating this guy Zach for four months, and I cared about him. But he would always say how much he was ‘in love’ with me. I’m a girl who strongly believes you can’t be in love in highschool, so I broke it off with him. And he was upset and started spreading such mean things about me and just being a jerk. But now.. I’m sorta dating his best friend. Now before jumping to conclusion, let me tell the whole story. So. I was hanging out with Alex and his friend Jack. (Jacks the boy) and I didn’t know he was Zach’s best friend. But I developed feelings for Jack, and he told me he liked me. I found out two days after that he was his best friend. Jack told me that Zach will get over me soon enough, and then we won’t have to hide it. But Zach is only mad at me, not Jack. And I kinda find that as bull, because it’s two people in a relationship. And I can’t help who I start to like. He told me once that “He can move on, but I can’t.” I was like “no, I’ll do what I want, you’ve controlled me for four months.” because he’s just always mean to me and I’m tired of it. But I just want to know.. Does it make me a who re to date his best friend? Or is it understandable?