People might see utilitarianism as cold, but deontology, in my opinion, is the colder one. They would justify passively killing billions for a clear conscience, whereas Utilitarianism only wants to maximize happiness for the many.
I feel like deontology doesn't care about the outcome; it only cares about the method. If saving a world requiring to kill one innocent person and only you knew about that, the right choice is to kill the person, but deontologist would say don't kill because some actions are inherently wrong no matter what. And if the world ended because you didn't kill that person, deontologist would say you did the right choice, it's not your fault, it's the fate of the world. Ultimately, I think it's literally prioritizing your clean hands and comfort from the judge over the goods of the many.
If you don't kill that one person in that scenario, you're actually worse than the guy who chooses to kill the person. You're effectively killing billions of lives for a clean hands. For the end justifies the means, for the genocide level of immoral one, is only justifiable if all other morally good option failed. If succeed, the immoral one automatically becomes wrong because there is the better way, and the goal has to be so positive and world changing, and only under extreme conditions otherwise it might not be justified.
But with one exception for the last resort rule: if time's running out, choose the one that has the highest success chance and the one that saves the most people and takes the fastest to complete overall, regardless of the method. For an individual murder or a few murder, like the trolley problem's kill two to save six, IT IS justified to kill two to save six. But for this one, it's an individual murder or a few murder, not a genocide, so for individual murder and a few murder it only gotta be so positive, it doesn't include world changing.
Also when doing a bad act (like lying or individual murders) for the greater good end during normal times, not only you should choose the one with the highest success chance, checking if alternatives existed, determine whether time is running out for the exception, or think is this a very positive net outcome, you should think, what happens to society if I do this? So you gotta think what impact does this act will do to society.
If time is running out and there's both immoral and moral ways, and immoral one has 95% success chance while the moral one has only 10% success chance, the logical choice is the first one. Although there's still a chance that the 10% actually works and the 95% fails, still 95% is still better even if there's a chance of failure too. And if it really fails, it is not a moral failure because we didn't know it would fail, whereas if the 10% succeed, you got lucky. You were still reckless because you were willing to let everyone die just for you to feel good about yourself.
I ever asked is killing one innocent live to save two murderers justified, but after I think I'm siding with the act utilitarianism for this one because those two murderers, if we choose to save them, can kill more people than two lives if we chose to save them.
Like if a train is headed towards one doctor that have a recipe of a cancer cure and he told someone he has it but he never wrote the recipe yet, and the other track there's 5 people, now both killing the doctor or the 5 is utilitarian, but I'm siding with saving the doctor.
since humans cannot 100% accurately predict the future, a morality of an act is judged by its EXPECTED outcome and true intentions, if a person killed 1 stranger to save 5 stranger and that 1 stranger was actually a cancer cure doctor, you didn't commit a moral failure, because how could we have genuinely known that? his expected outcome and intention was good, the expected outcome was more lives saved, the true intention was saving more lives. I'm trying to protect people from random bad luck that is out of our control and we couldn't have known it. if a person knowingly saved that one cancer cure doctor and the doctor died the next day from a car crash, it was a tragedy, a bad luck, the expected outcome and intention was good. But even if the expected outcome was at 10% success chance, it is still justified if it was the method with the highest probability at the time along with if the intention was good.
I generally agree to respect rules like don't kill during normal times because it'll make a good outcome and a happier future in the long term.