tbh one good point was made before about how to combat the KOS attitude and that was to give each character a certain skill set.
So for example someone may be better at repairing items, someone may be more effective at healing other people, etc so essentially you would end up with a group of people who may not have a type of character they need and would probably be more interested in finding that person to join their group.
But yes, KOS is a 50/50 probably and my problem has always been, if I don't kill them first then they will 99% of the time kill me. You don't trust anyone in these types of games