If a man does not benefit from being moral and ethical, why should he be moral or ethical?
If a man has more benefit being violent and ruthless why shouldn't he act in that way?
There is no quick answer because the question itself rests on too many axioms.
#1 - man seeks for his own greatest benefit
#2 - man will derive his greatest benefit through ruthlessness/ violence.
Assuming those two variables are true, and locking out the introduction of any other new variables makes this question a 'no brainer' because the answer was built into the question.
The first axiom would almost be universally granted. As much as possible, Man desires his maximum benefit.
Axiom 2 is universally countered that man achieves his maximum benefit when in a state of harmony. His next greatest benefit
could arguably be achieved through ruthless violence when he decides harmony is unreachable and seeks then not his maximum benefit but percieved second greatest benefit
. So the scenario breaks immediately on the first obstacle: "maximum benefit".
But the real fault in the question is the idea that man consciously knows what the maximum benefit is.
Man is capable of thought, therefore surmises the possibility
of religion or planes of existance beyond 1. For the purposes of our example the 2 are interchangeable because either introduced the possibility that maximum benefit here (plane 1) is not maximum benefit at all. Behavior then skewed according to beliefs designed, imagined, etc. Man seeks his maximum benefit by eschewing violence in the hopes that a logical Plane 1 harmony based maximum benefit will be attained, failing that that they personally
will be in the running for MB plane 2+. (Ergo they are seeking MB they just reject your axiom 2)
Recap: So why doesn't everyone behave selfishlessly (max benefit to self):
-Because we can't decide even for ourselves what that is.
-Because, on some level, we recognize that we don't know much about the universe and are individually yet collectively hedging our bets.
Which is why no ones a saint, because there may be nothing else, but few of us are absolute sinners either. Logically coupled with psychologically this behavior makes perfect sense. Those true saints and sinners among us are those who determined they "know" the answer to further planes sufficiently to place their bet "all in" if you'll forgive the coloquialism. Either by forsaking pleasure here or forsaking it 'there'.