I personally like the way they talk. It just feels like it fits. Although I have gotten used to reading odd speech patterns over my time playing games where differing dialects are actually shown in text. You get used to it after a while, the key is to keep in mind who you are speaking to. Once you get used to thinking in a certain dialect it gets pretty easy.

As for insulting and degrading NPC's I like them. If all the NPCs were all rainbows and sunshine there would be very little diversity. These kind of characters add to the overall world and it's society. That plus it makes sense when you take into account the general populations feelings about adventurers according to the lore.

The only thing I would change is add a fame system and make it so NPCs change dialog towards you depending on your fame. People don't typically insult you after you have helped them with a problem in their life.