What is the Buddhist perception of injustice, considering the law of Karma?

Considering the notion of our future being the result of our actions in the past, is there any room for the concept of unfairness in Buddhist philosophy? What is the right/skillful view one should take when witnessing suffering in a loved one? From my understanding, one should be compassionate towards the person and keep a calm mind as much as possible, but at the same time I find discomforting the idea that somehow, the present condition of a loved one has to do with his/her past actions, specially when considering more extreme examples of suffering without any apparent reason (e.g. childhood traumatic events, such as sexual abuse).