This is one of those topics I normally keep my hands (and mind) off because it tends to be a heavily value laden area of philosophy. While there are pragmatic arguments to be made for most things, they largely tend to be post-hoc rationalizations rather then true reason. The cornerstone of ethics and morality is trying to answer the question “what is good” and “what is bad”. From a realistic perspective, it is highly likely that as humans are social animals, morality and in-group behavioral characteristics that strengthen the group. Will have lead to those that have them out-breeding those who did not, and thus resulting in an in-born sense of morality, tempered by socialization.
Thus, if morality is merely the expression of instinct in regards to maintaining group cohesion, it follows that it can be ignored. However high number of group members doing so would result in lowered in-group cohesion, it follows that we would have to develop systems to enforce the mean of the behavior. These are our social methods for enforcing behavior criteria in a group, that are expressed in a multitude of social situations. From shunning, shaming, obligations, and fear of sanctions, these are all methods to enforce behavior and thought, just as some behaviors are used to both reward and incentivize pro-social behavior.
Some people are born with a strong internal locus of control, and are less influenced by the social methods of control. These are often themed contrarians, or iconoclasts. Some people are born with crossed wires in terms of genetics meaning they do not innately possess the pro-social behavior genes. Some, are even born without gene control and are immune to social control, these are sociopaths.
As humans love finding patterns and seem to love codifying those patterns even more, we developed systems in the field called “ethics” out of which there are two main forms.
This is a fancy word for “rule based ethics” the primary example of this area is the categorical imperative by Immanuel Kant, which states (paraphrase) “Act always as if your action will become natural law.” Which is to say that you should always handle yourself as if everyone else who gets in the same situation will act exactly as you did. This is a useful perspective, but it also underlines the limitations of this field of ethics.
The primary sources for deontological ethics are often religious texts such as the Bible (The 10 commandments, Golden Rule). One could argue that the legal system as well is a form of rule based ethics in inception, but not necessarily in practice.
The principle of deontological ethics is that whether or not an act is good, depends on whether it is according to the rule. The consequences of the act do not matter. For instance, if you have a rule that states “You shall always save someone’s life if you can” then saving Hitler’s life would have been a good act. Intent matters, consequences do not.
This style of ethics is a different beast overall, here only consequences matter, not intent. So, if you save someone’s life, and they grow up to be Hitler, then that act was a bad act. This ethical/moral position can be summarized in reductio ad absurdum as “The end justifies the means”. Based on my exploration of the subject-matter, I haven’t really found many philosophers espousing the virtues of this style of ethics. This may be due to its association with more “barbarian” and “Machiavellian” tribes in the past, that were perceived by the more civilized states at the time as not having rule of law.
Perhaps it has to do with the simple fact that most of us would undoubtedly be deeply immoral if the full consequences of some of our past engagements were explored. Even simpler, as consequences often cannot be predicted prior to an action, it has no prescriptive value. Or, the easiest of them all, it offers no control over a population and no way to punish that population for stepping out of line.
Which style of ethics should a modern man adopt? Deontological ethics will offer an easy to follow set of rules, but it would depend on how generalized the system is, and how expansive it is. A simple rule like the categorical imperative (in the formulation above) is highly general requiring a large amount of rational thought and consideration. However it will be adequate and adaptable to most situations. On the other hand, if the man selects a system such as Islam, or hardline Christianity, both of those characteristics will go down.
By selecting a consequentialist approach to ethics, he has the greatest flexibility, but no way to determine if his life is moral before viewing it in retrospect. One could even inquire whether a flexible moral system is even a moral system at all if the same act can be both immoral and moral depending on context. This is one of the more difficult aspects of Kant’s imperative in practice.
If a man who has had a string of bad luck, he is desolate, destitute, and depressed, isolated, irritated and inconsolable. If this man kills himself, then according to the categorical imperative, we need to imagine what the consequences would be if every man in his situation did so. I’m certain that many men have felt this way at some point and have gone on to live great lives once recovered. Therefore, it follows that this is an immoral act, due to the wasted potential. On the other hand, I’m sure many men have never recovered, and spend the rest of their lives living lives of quiet desperation. Therefore it follows that his act would be a moral one.
The dangers of morality and ethics
Morality and ethics are dangerous, they are by nature subjective and thus by adopting too stringent of a requirement for yourself, will put you at a disadvantage, on the other hand, not being stringent enough would most likely put you into the company of persons you perhaps do not want to be associated with. Illimitableman touched on this in an article on Machiavellianism and morality  where he writes:
Only intelligent men can really discuss the nuance of ethics and thus, whatever their disposition, cognisantly find a balance between altruism and sadism, principle and incentive. Of course if one is innately sadistic, only the discipline of volition can suppress such a thing.
This is the key to finding a moral system for yourself that strikes the desired balance of realism and pragmatism vs altruism and nativity. I use the term nativity here to mean “belief that human beings are inherently good“. A general belief that humans are inherently good will expose you to very high risks in situations where humans are not good, and makes you vulnerable to predators. A generally altruistic perspective, leads you open to be taken advantage of, from misguided faith in reciprocity. A high degree of realism, will make you prone to view the world through the law of the jungle, and when combined with a high degree of pragmatism, to the end justifies the means and “whatever works”.
The irony of this discussion is that we are left with the two things I started this discussion with, the combination of your genetics and your experiences that have shaped your life. Morality not being a fixed concept, it is merely a habit. If you are used to putting the needs of everyone else before your own, if you feel guilty when you say no or have been screwed over time and time again, you just need to train new habits. It will not feel good when you change a habit, it rarely does at first, but over time your life will improve.