if you tell me "you should do _ to maximize utility" I can reply "so what?"
If I tell you "you are morally required to do X", you can still reply "so what?". One can reply "so what?" to anything, and the fact that a moral theory doesn't prevent that is no objection to it.
(But, for clarity: what utilitarians say and others don't is less "if you want to maximize utility, do " than "you should do because it maximizes utility". It's not obvious to me which of those you meant.)
it's less plausible that I'm not obligated to maximize it to some extent
A utilitarian might very well say that you are -- hence my remark that various other "it is morally obligatory to ..." statements could be part of a utilitarian theory. But what makes a theory utilitarian is not its choice of where to draw the line between obligatory and not-obligatory, but the fact that it makes moral judgements on the basis of an evaluation of overall utility.
serial killer [...] 90% [...] 20% [...] 70% [...]
I think it will become clear that this argument can't be right if you consider a variant in which the serial killer's income is much larger than yours: the conclusion would then be that nothing you can do can make you better than the serial killer. What's gone wrong here is that when you say "a serial killer is terrible, so I have to be better than he is" you're evaluating him on a basis that has little to do with net utility, whereas when you say "I must give away at least 70% of my income to be better than him" you're switching to net utility. It's not a big surprise if mixing incompatible moral systems gives counterintuitive results.
On a typical utilitarian theory:
and the latter two points are roughly what we mean by saying he's a very bad person and you should do better. But the metric by which he's very bad and you should do better is something like "net utility, relative to what you're in a position to produce".
If I tell you "you are morally required to do X", you can still reply "so what?". One can reply "so what?" to anything, and the fact that a moral theory doesn't prevent that is no objection to it.
But for the kind of utilitarianism you're describing, if you tell me "you are morally required to do X", I can say "so what" and be correct by your moral theory's standards. I can't do that in response to anything.
Chist Hallquist wrote the following in an article (if you know the article please, please don't bring it up, I don't want to discuss the article in general):
"For example, utilitarianism apparently endorses killing a single innocent person and harvesting their organs if it will save five other people. It also appears to imply that donating all your money to charity beyond what you need to survive isn’t just admirable but morally obligatory. "
The non-bold part is not what is confusing me. But where does the "obligatory" part come in. I don't really how its obvious what, if any, ethical obligations utilitarianism implies. given a set of basic assumptions utilitarianism lets you argue whether one action is more moral than another. But I don’t see how its obvious which, if any, moral benchmarks utilitarianism sets for “obligatory.” I can see how certain frameworks on top of utilitarianism imply certain moral requirements. But I do not see how the bolded quote is a criticism of the basic theory of utilitarianism.
However this criticism comes up all the time. Honestly the best explanation I could come up with was that people were being unfair to utilitarianism and not thinking through their statements. But the above quote is by HallQ who is intelligent and thoughtful. So now I am genuinely very curious.
Do you think utilitarianism really require such extreme self sacrifice and if so why? And if it does not require this why do so many people say it does? I am very confused and would appreciate help working this out.
edit:
I am having trouble asking this question clearly. Since utilitarianism is probably best thought of as a cluster of beliefs. So its not clear what asking "does utilitarianism imply X" actually means. Still I made this post since I am confused. Many thoughtful people identity as utilitarian (for example Ozy and theunitofcaring) yet do not think people have extreme obligations. However I can think of examples where people do not seem to understand the implications of their ethical frameowrks. For example many Jewish people endorse the message of the following story:
Rabbi Hilel was asked to explain the Torah while standing on one foot and responded "What is hateful to you, do not do to your neighbor. That is the whole Torah; the rest is the explanation of this--go and study it!"
The story is presumably apocryphal but it is repeated all the time by Jewish people. However its hard to see how the story makes even a semblance of sense. The torah includes huge amounts of material that violates the "golden Rule" very badly. So people who think this story gives even a moderately accurate picture of the Torah's message are mistaken imo.