Hi Kaito Kid and welcome to PSE. You pose a thought-provoking question but my own view of utilitarianism and of what it allows, which I'll explain as best I can, is rather different from yours. I am not btw a utilitarian.
Just a conceptual point to begin with. Utilitarianism in its standard formulation is a requirement of maximisation - of the greatest (not 'greater') happiness of the greatest (not 'greater') number. If we replace the slightly nebulous 'happiness' with 'interests', 'preferences' or whatever, the key point about maximinisation remains.
Utilitarianism has always had at least in standard formulations a rule of distribution : 'everybody to count for one, nobody for more than one'. This is to say that everyone is equally morally considerable. It is a rule of equal concern (regard or consideration) not of equal treatment.
In your example, no-one else counts for anything but the sadist psychopath. The victim counts for nothing, nor do their loved ones or presumably the wider society, since 'the happiness he [the sadist psychopath] gains by doing it [torture and murder] is several orders of magnitude bigger than any suffering he causes to the victim and their loved ones combined'. The reactions of the wider society are not mentioned, I assume because they do not count.
But utilitarianism is an ethical theory in which what is sought is the greatest happiness of the greatest number or at least the greatest balance of happiness over unhappiness for all those affected. And all those affected have equal moral status, equal moral significance. That's a recap. The moral situation you describe does not meet these conditions.
A utilitarian can accept that from his own point of view the sadist psychopath is justified in doing what he does. By definition of his psychological state he cannot see that what he does is morally wrong or care about it even if he does see this. He experiences intense pleasure and this is all that concerns him.
It does not follow, though, that it is all that concerns a utilitarian, let alone that a utilitarian is obliged to endorse what he does. The utilitarian sees a much wider picture.
The following extract from Gardner Williams may help. There is no reason why we [utilitarians] should not accept that :
... there is nothing really shocking about the
truth that a sadist would be right from his own point of view in
performing his most horrible acts of cruelty if these satisfied him
most deeply in the long run. People are likely to make the mistake
of thinking that to admit a sadist might be right, from his own
point of view, if certain conditions were fulfilled, is an endorsement
of his cruelty. If he should be right in any way in torturing his
victims, ought we not to encourage or even emulate him? We
ought to do what is right, ourselves, should we not? And we ought
to encourage others to do what is right! But, of course, the truth
is that, from our own [utilitarian : GT] points of view, we ought not to encourage or
emulate a sadist. And we can not take any points of view but our
own. [We are utilitarians : GT.] What he does is obviously wrong from his victims' points of
view because they do not like it. It is a social wrong because most
people in society are horrified at it and suffer from it. It is wrong
for all who sympathize with the victims. It threatens humane institutions which people need in order to live satisfactory lives. If
successful, it would encourage other would-be sadists to indulge in
additional nefarious practices which would incapacitate or destroy
individuals needed for the support of institutions. ... [Everyone] must, if they can, restrain a sadist (Gerdner Wiliams, 'Hedonism, Conflict, and Cruelty', The Journal of Philosophy, Vol. 47, No. 23 (Nov. 9, 1950), pp. 649-656 : 654-5.)
The sadist psychopath violates the distribution rule : 'everybody to count for one, nobody for more than one' - his victim counts for nothing. And to assess the moral situation solely from a perspective in which the sadist's pleasure exceeds by an order of magnitude 'any suffering he causes to the victim and their loved ones combined' does not do justice to the utilitarian's concern for, not merely an immediate circle like this but 'the greatest number'.
Objections have been made to my answer :
Can you mention one utilitarian theorist, as opposed to a pedagogical presentation, who adopts the distribution role as you formulate it? (I confess I am asking because I am skeptical that there is one.)
The answer : 'Bentham, J.S. Mill' drew the response :
Where? This doesn’t strike me as fitting Mill’s rule utilitarianism at all. (E.g. hdl.handle.net/2027/spo.3521354.0003.002)
Mill clearly supports the distributional rule as I formulated it is : 'everybody to count for one, nobody for more than one' this when he says (J.S. Mill, Utilitarianism, 1863, ch.5 : http://www.gutenberg.org/files/11224/11224-h/11224-h.htm. :
...the Greatest-Happiness Principle. That principle is a mere form of words without rational signification, unless one person's happiness, supposed equal in degree (with the proper allowance made for kind), is counted for exactly as much as another's. Those conditions being supplied, Bentham's dictum, 'everybody to count for one, nobody for more than one,' might be written under the principle of utility as an explanatory commentary.
Perhaps I can better explain my position by distinguishing more clearly between equal concern (regard and consideration) and equal treatment. I read the distribution principle as a principle of equal concern, and it seems to me quite evident that Mill subscribed to it. There is to be equal regard and consideration for all affected parties. That is all I committed Mill to. I did not say or imply and do not believe that he is committed to equal treatment - equal shares or outcomes. (D.O. Brink, Mill's Progressive Principles, Oxford : OUP, 2013, 283-4.)
The distributional rule, as a rule of equal concern, is intrinsic to utilitarianism, in Mill's words 'explanatory' of it. It is not a rule, such as a requirement of honesty, promise-keeping, non-maleficence or even the Liberty Principle, which a utilitarian might adopt as contingently instrumental to her goal.
As for the point that 'This [the distribution rule] doesn’t strike me as fitting Mill’s rule utilitarianism at all', the distribution rule is (excuse the obvious) a rule even if as I have explained above a rather special rule. Why should it not fit ?