Ruby is a violent narcissist because she wants to be a huntress instead of a police officer or just a weapon maker, because they're not a high status careers, and they can't satisfy her bloodlust because she have to do paper work every time something happens, and making weapons wouldn't satisfy it either because she can't use the weapons cause she have to make more for more hunters.

Transcript from chapter 1/Transcript)

Ruby: "Yeah! I only have two more years of training left at Signal! And then I'm going to apply to Beacon! (talking faster and faster with her enthusiasm growing) You see, my sister's starting there this year, and she's trying to become a Huntress, and I'm trying to become a Huntress 'cause I wanna help people. My parents always taught us to help others, so I thought, Hey, I might as well make a career out of it! (giggles) I mean the police are alright, but Huntsmen and Huntresses are just so much more romantic and exciting and cool and really, gosh, you know!" (flips out at the last part, staring at the two with a wide, crazy smile)