● What are Witch Hunters?
The term is very general, but it refers to anyone who is trying to kill people with magic solely because they have magic. Originally the "Witch Hunters" were a split section of the church but modern witch hunters can come from any of many backgrounds and be part of many various groups or perhaps even a solo hunter.
● Why are they so dangerous?
Over the years the general idea of a witch hunter has gotten more and more deadly as they're able to get their hands on more and more weapons. But even if you don't take the weapons into consideration, witch hunters have crafted many ways to disable magic. In canon, a witch hunter named Karen uses a violin to sway the emotions used in magic and nullify spells, for example. There are also charms and anti-magic tricks that can disable a witch for just long enough to kill them.
● How common are they?
Luckily, even as weapons become more advanced the human law enforcement climbs in strength and capabilities as well. There may be many hundreds of witch hunters in a specific region but they do not typically attack a witch until they are 100% sure that they can pull it off. A lot of planning goes into properly murdering their target without getting caught. But when they do strike you can bet that they've thought through everything and that it'll be hard to get away.
● How do people become witch hunters?
Sometimes a family will raise their kids to take over when they're gone. Sometimes a person will come to hate magic and join up on their own. It depends on the character and what kind of group they're joining, or if they're about to go off on a solo mission.
● Can people with magic become Witch Hunters?
It depends on the case. Obviously they can if they want, but joining a group depends on if the group will forgive them or just want to shoot them on sight. So it's kind of a complicated question.
● What do they call themselves?
"Witch hunter" is a term the magical community came up with. The actual witch hunters can have many names depending on what they choose for themselves. However, most of them use "Witch hunter" for themselves as well. But once again, it really depends on the person and what sort of organization they're with.