Karma is a Sanskrit word meaning action. It denotes an active force, the inference being that the outcome of future events can be influenced by our actions. To suppose that karma is some sort of independent energy which predestines the course of our whole life is simply incorrect.
One thing you should remember is that mental transformations take time and are not easy. I think some people from the West, where technology is so good, think that everything is automatic.
The foundation of the Buddha's teachings lies in compassion, and the reason for practicing the teachings is to wipe out the persistence of ego, the number-one enemy of compassion.
Both these people illustrate the essential point that happiness is determined more by one's state of mind than by external events.
I think the person who has had more experience of hardships can stand more firmly in the face of problems than the person who has never experienced suffering. From this angle then, some suffering can be a good lesson for life.
External enemies, however brutal they are, only affect us during one lifetime. They have no power to harm us beyond this life. On the other hand, disturbing emotions are our inner enemies and can definitely cause disaster in future lives. These are, in fact, our worst enemies.