The only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others. His own good, either physical or moral, is not a sufficient warrant.
— John Stuart Mill, On Liberty
Mill proposes that the only thing that should limit ones liberty is doing harm to others. Let’s pretend that there was a society which was based on this principle, and only this. One might, and rightly so, immediately object to this society by pointing out what would be allowed in such a place:
Euthanasia or the killing of another at his own request, suicide, attempted suicide and suicide pacts, duelling, abortion, incest between brother and sister, are all acts which can be done in private and without offence to others and need not involve the corruption or exploitation of others.
— Lord Devlin, Morals and the Criminal Law
If my close friend and I get drunk an evening, and manage to get it in our heads that its time to do some pistol duelling between us just for fun, should this be allowed? We are both consenting to the situation, and yet no doubt one will end up being killed before the evening is over. Mill might have argued that this is a scenario of where ones action is harming another, even if there is consent, and that would include the killing of another at his own request too. Indeed, it does inflict harm to another, but wouldn’t this stance restrict our personal liberty to too much for Mill? After all, it is a consensual agreement between two grown citizens.
Let’s imagine he argues the other way then. Duelling between consenting adults is to be allowed according to the harm principle, Since it is consensual and voluntary, it is no concern of the state. Would such a state be preferable? A society where drunken duels are common, yet technically compliant with the harm principle, seems hardly ideal. Such a scenario seems more like a dystopia than a liberal ideal.
This is just one example where Mills principle leads to ambivalence, Lord Devlin brought up many more examples which I will not discuss for now. The point is, that in these cases, the principle is not directly applicable, and it is hard to see how to reason our way through the conundrum. The greatest hope is that the context of such events brings light to factors which tip the scale in one of the directions, but there is no guarantee that this is a solution for all such precarious scenarios.