Ed, your arguement is weak.
It's too late to hit this arguement while it's hot but this old chestnut deserves a crack because fundies dredge up the same ba dpoints over an over again.
Objective reasons on the wrongness of killing and hurting people:
#1) Instinct - human beings, being social creatures are able to identify with eachother and have an moderately strong instinct against killing eachother. This ingrained instinct is called sympathy or empathy. It's not fool proof and can be defeated by things such as a lifetime of abuse, a head full of bad wiring or military training, but it does account for something. You're right in saying it's not a totally rational or objective reason, but it is a reason that few can deny.
#2) The Law of Man - You kill people of hurt people, you get punished. Ah, you may say, why has this law been adopted by societies around the globe? Surely it must reflect some divine influence.
No, not really. Few societies are 100% anti-killing, otherwise they'd never go to war. You asked "why is murder wrong". The effective term being MURDER, defined as the UNSANCTIONED and ILLEGAL killing of a human being by another human being. There are numerous situations where a man can kill another man and without repurcussions or even be rewarded or esteemed for it (military service, corperal punishment, ritual sacrafice, ritual combat, ect.). A society must control who it kills and when. Why you may ask? The ability to contol killing is a basic, though underappreciated building block of society.
Lets say there are two hypothetical primitive societies the A-ians and the B-ians. The A-ians have developed a religious/philosophical or mystical code against unsanctioned killing. The B-ians have not. If one A-ian kills another, for whatever reason, the rest of the populace finds the killer and jail or executes or banishes him and the killing is stopped. If one B-ian kills another one of two things would happen he'll either figure that killing can get him what he wants, and kill again or the friends of the victim will find their own retribution. This begins the cruel cycle of vengeance, fracturing the society into numerous warring factions and eitehr destroying it, or making it weak enough to be conquered by other societies. That means if any ancient societies existig with a casual attitude about who can be killed and when, they didn't last long.
A similar connection between the recognition of property rights within a society and the ability to develop a strong economy would logically follow.
#3 Harsh Interconnection - Another, less tangible reason not to hurt people and avoid human suffering is that suffering is not a private matter. People who are hurt, people who suffer, people who are desperate will lash out and hurt others. You cannot exist above the suffering you create and if you make the world an uglier place, if you make people suffer, odds are someone will make you suffer in kind. It's not like there's any cosmic balance or karma or mystical explaination involved, it's just as simple as not shitting where you sleep and the fact that the people you step on on he way up are the ones you'll meet on the way down. On the other hand if you work to improve the world around you the people around you will be more mellow, more likely to help you out and less likely to punch you in the face when you walk by.
Now hedonism, even if not hurting anyone else, is tricky. If you do it wrong there are a number of associated self destructive paths. The only reason not to engage in balls-to-the-wall hedonism is a sense of self preservation and long term planning (along with a bit of pessimism about how 24 hours nonstop bliss is not all it's cracked up to be. Even boning hot chicks gets repetitive after the fiftieth gash in one day.) Living for immediate gratification is just a lack of planning and you'd learn that lesson as soon as you hit bottom, thus preventing any future binges of immediate gratification. Nothing like waking up in a puddle of hurl with a missing wallet and a case of crabs to instill a sense of long term strategy in man. Also hedonism costs money, unless you're a millionaire heir you have to work like a slave for it.
