How could this happen to me!!!

Umm no, he never regretted it. The quote is a translation from a piece of literature. It was about how he had to do his duty and he did his duty. He also admitted that could he go back he would do it all over again. Here's a great article: https://www.bbvaopenmind.com/en/science/physics/oppenheimer-from-the-atomic-bomb-to-pacifism/

The AK47