Umm, nope. He didn't work for 20 years on it just to be surprised it killed. He wasn't even against using it. He wouldn't even have changed anything if he could have gone back to do it again. Here's a very good article about him and his pacifism: https://www.bbvaopenmind.com/en/science/physics/oppenheimer-from-the-atomic-bomb-to-pacifism/
Interestingly, "never once said that he regretted building the bomb or recommending its use against Japan". He also told President Truman that he had blood on his hands.
Oppenheimer was a narcissistic sociopath who tried to poison his teacher over bad grades. I believe wholeheartedly he was on a massive power trip during the Manhattan project and that his famous line of "I am become Death" is a reflection of his ego.
Interestingly, "never once said that he regretted building the bomb or recommending its use against Japan". He also told President Truman that he had blood on his hands.