The truth, though it is an ugly truth is that, in order to perform dark magic, be it a curse or anything else there is a golden rule to follow. My father, and many Death Eaters lived by it and I heard them say it.

“You have to mean it”

You will never be able to KILL, never be able to TORTURE, never able to MANIPULATE unless you really and truly want to. The more desire you possess the more skill you own. That is how a Death Eater sees it. Skill is power.

Everyone has a weakness. We cannot deny this. From great wizards such as Albus Dumbledore and Lord Voldemort to snatchers and squibs. It is something humans just have. For many you could take the form of their Boggart but that really isn’t their true weakness.

For me, my weakness is that I don’t want to disappoint people. Others, certainly from Gryffindor have a weakness from friendship and pride. Other people have fiery tempers are overly ambitious or too trusting. This really could apply to anyone but in my experience… fear and love will bind you. They are not what control you.

Our strengths. Our weaknesses. These are used in our everyday magic. To perform the strongest of magic you need control. Believe it. Mean it.

The same principle is applicable to Vampires. In order to get rid of a vampire with a crucifix you have to believe in the magic, the power that it holds. So, in order to do anything. You have to believe it, you have to mean it.