Here is the article/chapter from Hare’s book Sorting Out Ethics. I think the book/article makes some really important points and that Hare is absolutely correct that the Kantian ethics and utilitarianism are taught is quite confused.
Despite that I think Hare misses the mark, somewhat badly, in framing the question in the way he does.
Kant could have been a utilitarian but to do so it would have to be a utilitarianism that makes use of Kantian style metaethics and it would have make the rightness and wrongness of actions based on the intention of the agent rather than the consequences. Hare says in the article, “It is still the intention to bring about the consequence that he is misinformed which makes telling a lie wrong.” And “people are judged by their intentions and not by the actual consequences” And the right kind of utilitarianism is hybrid rule/act utilitarianism where the normative principle is under the vast majority of circumstances a rule utilitarian principle based on preference satisfaction. The reason for this is as Kant says that we really don’t know how to bring about what he calls “the ultimate good.”
So in other words, Kant could have been a utilitarian if by “utilitarian” you mean something very very Kantian.
I think the issue better framed by acknowledging that when you start to address the various problems and weakness of utilitarianism, starting at the metaethical level and working your way to the normative level, you end up with something very Kantian. And of course it is important to note, as Hare does, that Kant thinks the consequences of actions are important and considers it one of our duties to promote the ends/happiness of others.
The most important point is that the Hare article makes is that the theories are very similar, when understood correctly. What I’m getting at is that when understand correctly theory we are left with is Kantian ethics, not Kant’s ethics but Kantian ethics.
So to the utilitarians out there: