Here is my understanding, when you act out to do negative things for the sake of short term/financial benefit, you will plant a seed which will result in a fruit which will come back to hurt you.
For example, you have a test tomorrow and instead of study for it, you go out party with friend, as a result you will receive pleasure of fun, in the end because of not studying you fail the test which is the fruit.
Thing that you have done to yourself are quite easily explain, because it often involves you and something else that is not a sentient being. What about things that you have done to another person who is also able to think and act?
For example if you harm someone for your self benefit, such as if you make a living by trafficking young girls into sexual slavery? You receives financial gain for now, but person that you traffic will feel much suffering as the result of your action. So in this case how will you receive the fruit of your bad action?
1. You will feel remorse for the evil you have done and it will haunt your mental being. This is what a lot of people say what will happen to you, BUT is this the only consequence you will suffer?
2. You might get caught by the police and be charge with a crime and serve out your punishment from jail. This is social justice and most likely outcome as well.
3. The victim that you traffic might one day catch up to you and harm you for what you have done to her.
4. You cover your track well and never got caught by authorities or someone who seek revenge, as a result in your next rebirth, you will feel the same pain which you have caused others in your previous rebirth.
BUT what is the karmic result of your action on your victims?
For example if you cause harm to another person by killing him, does that person will form the desire to kill you as what you have done to him? Like... in your next rebirth when you meet that person's rebirth, will that person be place into the position of someone that automatically hates you? For example... you being an American Marine and that person being the Taliban and you both meet on the battlefield?
Or when your victim rebirths, he or she will not suffer any resentment towards you, because you will somehow receive the karma feedback for killing him in other ways?
Basically, when you harm another person who have just as much right to exist as you do, how will you experience the fruit of what you done? And how the other person be effected by your action in the Karmic way?