i think there is not only one answer on your question. One aspect is, for example, if the individuum had a difficult childhood. This way they can evolve a narcisstic disorder. From own experience ( I know someone who used to be very dear to me) I can tell that those people often can't be taken responsible for their behaviour. In some cases I would say that they feel very insecure and in order to overcome such psychic stress they need other persons to see suffer, to make them suffer in order to think " yes, I am better than him/her". This boosts they self-confidence, otherwise they would go under. Such problems can often make interpersonal relationships extremely complicated to unbearable. Maybe the next time just try to have a calm talk with such persons, but as I know, they will never learn nor try to change their behaviour, because in their eyes, they don't see what they are doing wrong.
I could talk a lot more and explicit but I hope this gives you a bit insight.