Often in the discourse between the irreligious and religious the topic of morals is brought up. The most common question put forth by religious people towards secularists is the idea that without a moral law giver (GOD) morals wouldn't exist. Thereby following that previous question with, if god doesn't exist where do you get your morals from or what stops you from murdering other people.
There are many different ways to approaching the problem of solving where our morals come from and I shall be covering several aspects of the moral argument.
Evolutionarily it is hard to think up if not impossible to imagine a species that could have evolved with a moral code of conduct that would permit the murder of their fellow members of their species. By looking at nature we would be hard pressed to find a species that performs murder of its fellow in group allies. What we see in nature are battles for territory, sex, food, and position which rarely lead to the murder of the participants. Battles are wrought in a variety of ways such as the innocence of a peacocks tail to the violent clashing of rams horns. A traditional view of the evolution of intraspecific battles are circled around the idea of ESS (Evolutionary Stable Strategies) opportunity costs and utility. If a specific player were to attempt to murder another, the cost of the murder will affect whether such a behavior will be passed down to that performers progeny. The cost of murder is obviously high in most cases, especially within the animal kingdom. If a murder is attempted the victim will most likely fight back, serious injury could occur or even death, revenge comes into play in human societies which could lead to endless vengeance and rivalry. Looking back into the evolutionary history of mankind any type of injury occurring outside of the past 100 years would be life threatening. If murder were performed before reproducing and resulted in the murderer being murdered in turn, those genes for "that behavior" (behavior and the linkage to genes is not a simple matter but the link between genes and behavior has been established) would simply not be passed on.
We can look for examples of intraspecific "murder" in nature for a better understanding of when and how it takes place. A female prey mantis can be commonly seen biting the head off of its male mate during sexual intercourse. This act of cannibalism has been shown to boost the sexual potency of the male conferring a big selection pressure on the practice. Wars between rival insect factions have been seen in nature, but such events can hardly be called senseless murder. The closest example that compares to human murder can be found in our nearest relatives the apes and monkeys. Chimps have been seen to brutally murder children of rival alpha males after taking power and have been seen murdering in group dissidents. The cost of murdering rivals children is low as they cannot protect themselves and after such murder has taken place the females begin to ovulate allowing for sexual reproduction to occur under the new alpha male. The murder of dissidents in chimp culture can be seen as a way to maintain the power structure. To my knowledge there are no known cases of serial killers in ape societies other in humans.
I will appeal to common sense and ask the reader to imagine a species that freely commits murder of its fellow kind and imagine if such a species could have survived the war of nature without succumbing to extinction.
Now on to the aspect of how our genes can control our behavior. Genes affect behavior in humans through the reward systems, empathy centers, mirror neurons etc.. (performing a deed can make us feel good or bad internally) Most people have a tendency to feel queasy when even thinking about killing other human beings. The exception to the rule are psychopaths who feel no emotions towards others due to brain defects either genetic or onset due to some form of brain damage.
Since genes have been selected against murder it is easy to see that humans are hardwired against performing murder in normal circumstances and are internally awarded to do so. Now that I have described how the moral sense against murder has been evolved naturalistically I want to continue on to the subjective realm of morals.
Laws are put in place to deter criminal acts. If a person were to commit murder there is a sufficient punishment that will be placed upon that individual if caught and convicted. A common issue with people is to say that "you are not moral if all that is keeping you from murdering is the fear of punishment", which is valid but detracts from the issue. If there were no punishment for murder would the murder rate go up, down, or stay the same? I doubt that I would have to argue that the murder rate would rise significantly. Obviously the laws and punishments are sufficient to keep a certain number of people from murdering.
The cost of murder is high, as the murderer risks severe injury or death by performing their actions. What are the benefits of committing murder? In a situation where someone was going to murder me and I had to protect myself in self defense I would kill the attacker, thereby giving me the benefit of life.
I would ask the reader to answer the question as to whether or not they would not commit murder in any and all situations and consider whether murder in self defense is moral or not. Most religions sanction certain forms of murder with the exception of Jainism and other pacifistic religions, so a devout christian or muslim could not claim murder as being immoral in all circumstances. State sanctioned murder occurs in several forms, such as self defense, war, and capital punishment. To address the extremely pacifistic religions, I would like the reader to think about this statement "a lack of action can be just as immoral as an immoral action because of the opportunity cost of inaction." A simple thought experiment can be performed here to show my point, if a known terrorist had access to a nuclear bomb and set it up in new york and had the trigger near him but you had the chance to kill him before he would take the trigger and end the life of millions, what would be the correct moral choice of action?
I have not addressed whether or not murder during war or capital punishment are moral but that topics need to be looked at individually and are not needed in this particular argument.
To answer the beginning question as to where I get my morals from and what stops me from murdering others. It is simple, all the factors that I have outlined above. By our nature we are hardwired to avoid murdering others, there are significant costs to committing murder including risks to self, a society could not function and would most likely lead to extinction if murder were permitted freely, and in some circumstances I would murder if the benefits far outweighed the costs, an example being self-defense or in the defense of others.
I don't want to typecast as a utilitarian but my morals do tend towards utilitarianism.
I am sure that some things have escaped me while I wrote this so I may add addendums to this as new ideas come to mind.