The Halo universe is full of choices between greater goods and lesser evils; from the Forerunners choosing to let themselves be wiped out by the firing of the eponymous Halo rings for failing the Mantle of Responsibility during their war with the parasitic Flood, to the Elites and Humans choosing to put aside their bad blood to stop the Prophet of Truth at the end of the Human-Covenant War, to the Flood, the most powerful, most evil force in the galaxy, choosing to side with the allies to prevent the firing of the Halo array by Truth. Players of the franchise are no strangers to these choices. However, it's outside of the games that the greatest test of this choice is found.
"The road to hell is paved with good intentions", this quote personifies the Office of Naval Intelligence. Officially, a branch of the United Nations Space Command that handles and analyzes intelligence data, the reality of ONI is far more sinister. While ONI does actually deal in intelligence, that is only one section of the organization. ONI has several sections, and the further down the ladder you go, the more corrupt it gets. ONI Section 3 is one of the most notorious of its divisions, infamous among those who know it exists, for its heinous actions. It is most famously known for being in charge of the SPARTAN-II program, which, under the direction of Dr. Catherine Halsey, kidnapped over 70 six-year-old children. The reason being that Dr. Halsey had determined that genetically modifying children would be easier than doing so to adults, and ONI not only allowed her to do so, but also helped her to cover it up by providing her with flash clones of the kidnapped children to trick their parents. Making matters worse, the flash cloning process was imperfect, and almost all of the cloned children died within a year. Why was all of this necessary? ONI wanted Dr. Halsey to create super-soldiers, so she genetically modified the children and brainwashed them into becoming soldiers. Even more disturbing is that the training and genetic modifications ended up killing or crippling over 30 of them.
That's right, the greatest heroes of humanity, the protagonists of most of the Halo games, are brainwashed, kidnapped, forcibly genetically modified child-soldiers. But one generation of super-soldiers wasn't enough. The progenitor's to the SPARTAN-II project, the ORION project, lead to almost all of the adult volunteers becoming near-sterile and gaining genetic deformities, and ONI covered that up too by creating a fake disease. The SPARTAN-II's were elites, but they were too few in number to change the losing war against the alien Covenant. So ONI took to rounding up children orphaned by the Covenant, manipulating their fractured psyche's, and genetically modifying them as well. These children became the SPARTAN-III's, and this time ONI kidnapped hundreds of them. Many of them were sent on mass suicide missions as a cheaper, but still effective, version of the SPARTAN-II's.
It could be argued, however, that these actions were justified due to the state of war. The massive civil war that Humanity faced before hostile contact with the Covenant, and then the extinction-level threat the Covenant posed (even leading to the UNSC taking formal control of the government), that any action to save mankind would be justified. However, ONI's actions go beyond the war. Even after the Human-Covenant War ended, and Humanity and the Elites became true allies, ONI's undercover actions continued. They covered up the flood invasion of Earth at Voi. They grew a poisoned crop of the Elite's main food supply, that could easily outgrow the regular crop as a contingency, and they covered up the truth about the SPARTAN programs and put potential whistleblowers into prisons with such isolation that it sent them insane.
This is just a taste of some of the corrupt actions of ONI. The real point of interest is the fact that the men and women in charge of ONI see themselves as always doing the right thing, and that their actions are for the benefit and survival of mankind. So, my answer to the question the title poses, are these evil actions taken for the greater good acceptable?
I'm forced to begrudgingly accept it. The SPARTAN program saved humanity, ONI's contingency plans could always become necessary, and covering up the existence of the flood was necessary to prevent our idea of humanity as we know it and our society from collapsing. I don't like it, I don't agree with it, but I accept it. I guess that means I'm a utilitarian at heart. The lore itself seems to hold disdain for the actions of ONI, but still, their actions did help to save humanity. If I was to change anything about ONI, I would force it to have more oversight, but the clandestine nature of much of their operations makes that almost impossible.
Now let's hope this article doesn't make ONI decide to have me commit suicide with two bullets to the back of the head.