Karma is the result of our action, what we did and what we received. Karma is not something visible that everyone can see. But not able to see it doesn’t mean it does not exist.
People who does not believe the existence of Karma, is just like they don’t believe the existence of electricity, why? Because electricity is also not something we can see, even though Televisions, computers, washing machines, fridges, etc, etc is already the prove of electricity, this is the fact, and it is a undeniable fact, but if there are people that still insisted that electricity does not exist, what can we do?
When someone that is so insist on denying something, there is only one explanation, fear. It is because of their fear on something, they are afraid of facing it, so they deny, they refuse to face it even though it is already right in front of them. So, if they accept what karma is, they have to step out of their current ‘comfort’ zone. They are afraid of change.
With such action, it is just a pure proof of ignorance and selfishness.