Medical doctor who tried science now wants to quit Phd and science altogether. How to explain to supervisor?
There is no problem here. Just inform the PI that you intend to leave the program at the earliest opportunity. You haven't signed a contract with the devil for your soul. People change, people grow, people learn. Others understand that. 

It would be good, however, for  you to do what you can to leave things tidy when you go, not leaving others (PI, other students) in the lurch. To do this effectively you may need to spend some time cleaning up the current work and either finalizing it or passing it on to another. That is courtesy, but useful to you for the future if you decide later to re-evaluate yet again. 

Take care of yourself; your future is yours and yours alone.