Margaret Thatcher probably had it right during her time when she said,"A world without nuclear weapons would be less stable and more dangerous for all of us". Joe Stalin, Chairman Mao, and probably other despots would have used their expendable populations to expand their borders and power and with the increased lethality of newer "conventional weapons", there would be fewer of us on this forum. As to crazy Muslim leaders with nukes Thatcher would have it wrong; Mutually Assured Destruction (MAD) might be what they want to create their end times scenarios.
As to Obama, if he regrets the use of nuclear bombs on Japan, that would also mean he regrets that the Japanese still exist. If we invaded or just blockaded Japan and let Curtis LeMay dump conventional bombs on the Japanese, it would have been genocide. And again, there would be fewer of us on this forum because we wouldn't have been born.