When it comes to murder, I think it's important to differentiate between murder and killing. Murder is never, IMO, justifiable while killing, though regrettable, may sometimes be necessary. Had the police been given the chance to kill the Virginia Tech shooter before he murdered 30+ people I believe it would have been the moral thing to do if there'd been no other way to stop him. Regrettable, but moral.
As for the issue of sex between the age groups, it's not as ancient as we might believe. There is, or was within the past century I believe, a tribe called the Sambia in New Guinea that, as part of the coming of age ritual for young men, had younger boys perform sex acts on the older boys. I won't go into great detail so as not to post anything too offensive. I'll just say that in our society this would be unacceptable and probably illegal while in their culture it's a way for one generation of men to pass their manhood on to the next generation.
I'm fascinated by the relativity of morality and hope your question spurs some thought and discussion.