"morality", as in the concept, is not simply "something built by others". Morality and ethics deal with how it is proper to interact with eachother, we can each have unique ideas of how morals work, or of what is moral.
Sure, society does have certain morals it tries to enforce, but by rejecting those, you have not rejected the concept of morality altogether, you have simply adopted a different code.
You describe before how you do agree that killing and stealing are wrong, though you believe circumstances might mediate this in a utilitarian manner. That is a moral stance, not an amoral stance. For it to be amoral, there would be no consideration of the moral implications of killing, and the question would simply be of a cost/benefit type.
You are presenting a false dichotomy here. Simply rejecting "society's morals" is not the same as having no morals. Everything you have said so far indicates that you do see why killing innocent children is wrong, not simply non-profitiable.