"Morals" is just a word for the desire to not kill a member of your own species. We're not alone in that as very few animals murder their own species. In fact, few animals kill anything unless they have to, and I will include humans in that statement.
However, there are always people who are NOT like this. They're sociopaths and they generally don't give a damn about other people. These guys (because sadly it's usually males who are sociopaths, not exclusively, just mostly) couldn't give a flying fig about anyone else. Not in an evil way, more in a Dexter kind of way.. They just don't see other people as important. Much the same way as a normal person would treat a fly. We wouldn't go and try to kill a fly, but if we swatted it to death, well, we wouldn't care much about it. I mean, it's just a fly.
I've read that 5% of nearly all mammalian species are gay. In the same line, nearly 1% of all mammalian species are sociopaths.. This isn't necessarily a bad thing as evolution requires as many physical and mental traits as possible (within reason) to give any species a chance of survival. As far as humans go, we're damn good at finding work for societies sociopaths. We put them in the army. I need to check my facts here but I seem to remember reading something that mentioned in the American Civil War only about 1 in 10 soldiers would actually do any killing. The "normal" guys just didn't want to kill anyone. In World War II it was much the same, reports of soldiers emptying entire clips into the air so the wouldn't kill anyone. Once this was noticed the recruiting was changed and apparently armies are now up to 90% sociopathic. Hell, we've all seen the videos of gunship pilots laughing as enemies are cut down, or seen the news of atrocities in prison camps etc.
Normal people like myself and 99% of everyone else who reads this would not do that shit. Not for any reason - the same way that 5% of the people who read this are going to be gay.
Anyway, I have gotten away from the point. You mentioned that reprisals might be a reason why you don't turn around and murder your friend. Well, that's a very good point. If you killed your buddy then you would be seriously ostracised by everyone else, because lets face it, who wants to hang around someone who might kill them? But the main reason you don't kill your buddy is that you just don't want to. They are not a threat to you or your loved ones in any way, so there is just no point. You may as well ask what is preventing you from knocking down your bedroom wall and replacing it with ham. What would be the point?
