When we indulged in Bhagavad Gita with a pre-understanding that God exists and so does world of souls atmans then we can clearly understand the law of karma in totality.
The precept fruits of karma simply mean that whatever human beings did, the fruits of all indulgence, entire karma belonged to the actual doer which is our soul atman, spirit existing in heart. The manifest human form was never the doer.
A soul atman in its cosmic sojourn manifests human form to cleanse self of dross impurities within. The human form manifested by soul atman is only a means to an end, for spirit within to indulge in karma. By self no soul atman could purify itself and needs a body, a medium to work out its karma. Our soul atman existing in heart is the real master and controller of human body.
As per inscrutable law of karma as we sow so shall we reap, all the fruits of karma indulged by human beings belonged to the real owner spirit, consciousness existing in heart!
It is only prompted by ego, human beings think they manifested a soul atman within and most even did not believe in existence of souls’ atmans. Demolishing karma is goal of spirituality. Indulging in Bhagavad Gita human beings not only demolish ego but also succeed overpowering five senses and mind forever. The prime goal of indulgence in Bhagavad Gita is demolition of ego. Only then we could realize we were not the doer but only a means to an end.
From six years of age do not know how I lived like a trustee, as did JRD Tata and also King Janaka (father of Sita in Ramayana). I also never attached self to fruits of karma performed. For me the doer always was God Almighty himself, Brahman in Hinduism. I was only an instrument doing his bidding all the time.
If we believed in concept of trusteeship and also followed it then the precept of fruits of karma becomes clear in entirety.
Everything visible in this world was not a creation of mankind. Everything truly belonged to God. We even did not create self; we were just born not of our efforts. We came unto this world not of our efforts and when we shall leave this world, we shall not be able to take anything to other world. Everything manifest would be left behind. We came alone and shall leave alone.
But this simple logic of life is not understood by most human beings on mother earth. Dominated by a bloated ego human beings think they were the owner of all things manifested by them. It is at this stage Bhagavad Gita comes to our rescue but only for seekers, travelers of spiritual path never the common man, masses.
The most essential principle of spiritual life, offering fruits of karma performed to God Almighty is following concept of trusteeship in words and action.
In the manifest physical world, fruits of karma always belonged to the individual self.