J. Robert Oppenheimer, "father of the atomic bomb," had very complex views on the subject. One might split them up into sub-topics:
On his role in the Manhattan Project, which involved the creation and use of the bombs: he never, ever, expressed anything like serious regret about what he had done during the war. He did, at times, express that he felt it had come with a great responsibility, and that there were certain matters (e.g. his role in the choose of cities to target, and his recommendation against any kind of warning for Japan) that he seems to have not been terribly comfortable with over time. I wouldn't go as far as "proud." He saw things in terms of his "duty" (that is what the "now I am become death" quote is about), in terms of his "role" to play. As he put it in his 1954 security hearing: "When I and other scientists were called on for advice, our principal duty was to make our technical experience and judgment available. We were called to do this in a context and against a background of the official views of the Government on the military and political situation of our country." Which is to say, when talking about his work he often phrased it as a duty to a country, as opposed to something he had any role in initiating, responsibility for making policy about, etc. It is noteworthy that almost immediately after the war ended, he resigned from this role — he saw himself, as he later put it, "the directory of an emergency." It was not a role he wanted to have outside of the war.
On his view about nuclear weapons in general, Oppenheimer had hoped they would become an impetus for the creation of a better world. He believed that they imperiled human civilization in a new way, were a scientific and technological achievement that would require a reevaluation of traditional modes of international politics, and perhaps even human ethics. Even during the war he had started the seeds of this, but he devoted himself to it in the immediate postwar, the idea of "international control" of nuclear weapons. The basic gist was that an arms race would be the most dangerous possible outcome of the invention of the bomb, and that instead the newly-formed United Nations should use the threat of nuclear war as a motivation for creating a truly stable system of international order, one designed around freedom of exchange and expression, with scientists leading the way. The goal would be to prevent the development of nuclear weapons globally (including, eventually, in the United States). The result though, he knew, would go far beyond that — it might radically transform the world. He threw himself into this for 1946-1947. It ultimately was not successful; the Soviets were not especially interested, and the United States' officials were only lukewarm to the idea. But the incident sheds some light on what Oppenheimer's feelings about the bomb were at this point — a means to an ends, perhaps, a scary, terrible thing that might, however, lead to some good. As he famously put it in November 1945: "If atomic bombs are to be added as new weapons to the arsenals of a warring world, or to the arsenals of the nations preparing for war, then the time will come when mankind will curse the names of Los Alamos and Hiroshima. The people of this world must unite or they will perish."
From 1947 until 1952, Oppenheimer served as the head of the General Advisory Committee to the Atomic Energy Commission. That was a prominent advisory board within the American nuclear complex. In this position, Oppenheimer's overall approach was fairly "conservative" from the position of policy — he sought to do his duty, which as he saw it, in a post-international control world, as encouraging the United States to develop weapons and policies that would avoid world war. His major initiative here involved a focus on tactical weapons (small nuclear bombs that would be used against tanks or military facilities) and away from city-destroying weapons (like the hydrogen bomb, which he opposed developing in 1949). One can argue whether that approach was right or wrong (it is not at all obvious whether tactical weapons vs. strategic weapons deter better or worse, make the world more or less stable), but it's a definite position, and one that reflects a bit, perhaps, on his overall feelings about the weapon. Ultimately, though, it is worth noting that his position was not one of a dissident, not one of a "dove," not one of an anti-nuclear activist, etc. He worked for the government, he encouraged further nuclear weapons development, he encouraged developing weapons that he felt might actually be used — but ideally against tanks, not cities.
In 1953 Oppenheimer attempted (unsuccessfully) to spearhead an effort to better rationalize the US nuclear policy, which included being more openly frank with the US public about its possibilities and limitations (Operation Candor). This was not successful, in part because Oppenheimer was falling out of favor with the US government, and also because the Soviet "H-bomb" test of 1953 seemed to encourage "hard-liners" with regard to the USSR. It did, in a roundabout way, lead to Eisenhower's "Atoms for Peace" policy, but it is not clear that this was really what Oppenheimer was after. In late 1953, Oppenheimer's security clearance came under scrutiny on account of enemies he had made in his previous work (his position on the H-bomb made many of them), and in the spring of 1954 he had a lengthy and embarrassing security hearing that resulted in him being stripped of his clearance. During the hearing, it is worth noting that he consistently defended all of his efforts and policies as being in the interests (as he saw them) of the United States and the world, and he never repudiated his work on developing or even using the bomb (though he did indicate some of the previously indicated ambivalence). After his hearing was stripped, he became quite a bitter person and spoke very little of nuclear policy matters, but he did at times (esp. in the 1960s, when he was dying) express regret that international control had not been successful.
None of the above in my mind adds up to an easy one-liner about Oppenheimer's feelings about the atomic bomb. They were complex. They were, at times, contradictory. They were not repudiating or rejecting, they were not embracing and lauding. He was not in the "thank God for the atomic bomb" category, nor was he in the category that saw them as unadulterated evils. He had complicated feelings about the relationship between technical advisors and political decision-makers and military personnel, he saw the atomic bomb as something perhaps unavoidable but presenting distinctly different paths forward. He ultimately felt, I think, regret about what he had been unable to accomplish — his goal was never to be a mere weapon-maker. But he never said he regretted making or using the bomb.
In 1966, not long before he died (and he knew he was dying, of throat cancer), he wrote a note to a former graduate student of his that I have found useful in thinking about his stance:
What I have never done is to express regret for doing what I did and could at Los Alamos; in fact, under quite dramatic circumstances, I have reaffirmed my sense that, with all the black and white, that was something I did not regret. ... My own feelings on responsibility and guilt have always had to do with the present, and so far in this life that has been more than enough to occupy me.
The note is in reference to a play about Oppenheimer that made him seem like a regretful martyr — that was never a role he appreciated being cast into, though it has been a very persistent way for people to read his saga.
The best Oppenheimer biography out there at the moment is Bird and Sherwin's American Prometheus — it won the Pulitzer Prize for good reason. I am also partial to Carson ed., Reappraising Oppenheimer, which has some very nice essays from many different scholarly takes on Oppenheimer's complicated career. Monk's Oppenheimer doesn't break any new ground but it is very nicely written and has some perceptive insights into his character formation.