it all comes down to relativism
for your example, you list a serial killer.
A serial killer commits murder against humans in a non socially constructive fashion. I ASSUME (don't know) that the reason this is wrong is because it threatens the integrity of the social order.
The same serial killer could be a soldier & be given medals for committing murder. Or he could be born in ancient greece and like killing his slaves, in that case it would be ok too. Or he could be an exterminator in this age, killing non humans. that is considered ok too.
To me, his killing is wrong because it threatens what we want, which is what emotions are. what we want, what is important, what isn't.
right & wrong are just a result of emotions anyway. And emotions are basically arbitrary in existence (not arbitrary in action, but their existence is arbitrary). In essense, we are stranded on a rock, tricked into unnatural perspectives to service a highly amoral purpose (evolution) in an emotionless existence. Hard to explain what i mean. evolution invented emotions and evolution isn't universal (i don't think).
I don't consider something wrong unless it threatens the status quo or causes suffering. But even that is just a signal that i am a good lil' citizen, only concerned with suffering that affects the survival of the social order. I don't care too much about people in the 3rd world, where a $15 donation could prevent a lifetime of disease.