This trust system is put in place to discredit scammers and protect other users. Especially on this forum negative trust is a big stigma on one's account and you no one will trust you or go first in a trade. So it's something you want to avoid.
But I have also seen users getting negative trust by acting disrespectful to other individuals.
As much as I understand people who give others negative trust because they are being a douche or a pain to others. I strongly believe that it's not just to do that simply because someone being arrogant or disrespectful doesn't necessarily correlate with their trustworthiness. Therefore no one should receive negative trust for being a douche. I do not promote being a douche or a pain not by any means but someone can be the biggest douchebag but still be a trustworthy person that always keeps his/her word when it comes to trading.