You're talking about a job that wears black plate armor with spikes, wields an abnormally large spear, and has the word "dragon" in its name. It's just one of those "cool" factor jobs like Ninja that are notorious for attracting all sorts of bad players.

Don't worry though, Dark Knight will suffer the same fate. Only difference is that there will be much more rage since bad DRKs will cause the whole party to kiss the dirt.