How do you justify any set of morals?

Lets say I ask someone why it's wrong to kill another person. The first thing they will say is "It's just wrong." To which I say "why." They'd probably say "because society can't function if people are going around killing each other." "Ok, why does society need to function?' "So we'll have food, protection, and culture." "WHY?!" Basically, I could keep going on, and there would never be a subjective reason for not killing. And if morality is arbitrary, why can't I go on a killing spree getting rid of people I dislike?