The majority once supported the institution of slavery. Morality means NOT HARMING OTHERS. If you can convince most people that something immoral is moral, it's still immoral.
Rather than "right" or "wrong" it would be impractical. Morality is a hard thing to tack down with any simple definition. In general terms, it stems from our innate nature to have empathy for others (within our own groups at least), which makes living in groups (which is safer) more feasible.
Of course everyone in any particular group would have to come to a consensus on what the rules would be. This is why we have laws. Here's how I imagine your scenario would turn out: If murder was made legal (for some weird reason), our country would probably break down into smaller tribes or clans of trust.
Within these smaller groups would be an agreement among the members to not kill each-other, for survival reasons if nothing else. Now continue this scenario for a bit. Tribes and clans would eventually make alliances with others, providing more safety and resources for all concerned.
The bigger tribes would flourish and eventually we would have a single governing body where murder would be illegal. It's a survival issue. We individually have an aversion to murder because that trait is what has allowed us to survive.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.