Christians will tell you that you don't become a Christian merely by slapping a label on yourself. You're not a Christian simply because you SAY you're a Christian.
The word "Christian" either stands for something or it doesn't.
Imagine a person who says, for instance, "I don't believe in the resurrection of Jesus, but I'm still a Christian." No, you're not.
Or a person who says, "I sacrifice animals in my rituals, and it's okay to kill people who believe differently, but I'm still a Wiccan." No, you're not.
The term "Pagan" (or "witch" or "Wiccan") is more elastic than the term "Christian," but there is still a sine qua non. Words have meanings.
The problem is not with Paganism, which embraces many things. The problem is with Christianity, a religion which is by its nature oppositional and non-inclusive (John 14:6) and hostile to witches.
You have the right to believe whatever you want to believe. You also have the right to call yourself anything you want to. But people who have a basic understanding of what a Christian is and what a Pagan is might not take you seriously if you say that you're both.