Game of Thrones is set in a world of pseudo-medieval feudal states and the behaviour of the characters is based on a morality similar to that of the real European medieval period. In that era concepts of honour and loyalty, family ties and keeping ones word/oath/promise were taken much more seriously than they are today. These concepts were closely tied to to a person's social standing and so for example breaking a promise could seriously lower your social standing, and bring shame on your family.
Regarding the amount of killing, there are certainly some characters in GOT who would qualify as sociopaths today, but these are also characters living in a world with different morality standards from our own where death from disease, injury etc. is far more common and life is generally "cheaper". Killing in self defence, or defence of one's honour or family, or for more abstract concepts like "glory" or "duty" is also far more accepted. These social and moral traits are accentuated at the top level of society, which is the focus of much of GOT, where the risks and rewards and competitiveness are all that much higher.
So the characters for the most part are just behaving in line with the fictional society they are a part of, which itself is loosely based on the social and moral standards of medieval Europe.