act utilitarianism has no limit. it isn't about desert or obligation but simply fulfilling collective desires whether or not those desires are earned or deserved. the minority might have a legitimate claim to their own happiness (for example, they want to keep owning their property because they worked hard to gain it while the majority did not) but bentham's formula simply says that simply because there are more people in the majority means that they can do whatever they want to the minority so long as there is a technical excess of happiness created and a deficiency of sadness net. so act utilitarianism is very impersonal and void of any kind of legitimacy-creating context.
mill's formula (rule util.) is better here because it is more kantian and universal to individuals - you can't just treat people like pure tools and as means to an end or else anbody is potentially merely a slave to a larger group, which isn't deserved. if you treat one person in a certain way, you must treat everybody like this. the rule of these rules is that the rules themselves are procedural, not substantive - if you follow the procedures which would be thought of to produce generally (not all the time, which is a limit) happy result for the society as a whole, then it is better to have rules that go against brute act utilitarianism because if nobody followed rules that lead to that general happiness then you'll have a society of disorder and mistrust (etc) which is not stably a utilitarian society. so let's say that somebody worked to earn their house that the mob want to steal for their own larger happiness - a universal rule that it is wrong to steal would mean that people can all have an incentive to work which boosts an economy (which is a greater good for the greater number), and they would still be able to enjoy their house without the constant worry of it suddenly disappearing. so mills' theory factors in fairness and universality, we can say, whereas bentham's theory is a "pig ethic" because it can justify piggish and uncivilised acts; for instance, "9/10 enjoy gang rape"...but does the rape victim deserve to be raped based on their deeds in society? are they a slave to the greater number just because they can give them happiness that they aren't necessarily deserving of?