It's a fair question. Answering it well is a fundamental start for the topic of ethics.
I was thinking about this in terms partly of politics - right-wing politics decides on certain things it finds important even if they result in less wealth, less freedom for people, even to the point people lose their lives. That's true both of domestic and foreign policy. Many times the US has supported policies to gain wealth, gain power, even for reasons like 'projecting power' or 'rallying domestic support' (think Grenada), which take lives.
Also in terms of abortion - which I choose not to discuss here, but if you grant the position that life begins at conception, you are faced with the question about the OP's question, the right and wrong of killing the early person still just cells growing - is that right or wrong? What if it's to create a grown person with in vitro?
Human history is filled with killing, actively and passively, and related questions such as providing help for people - food, shelter, medical care.
Today many thousands of Americans annually lose their lives without medical care. We can afford care for them. What's the right and wrong?
Should we protect Mitt Romney avoiding taxes to have another $10 million not paid in taxes as more important than providing universal healthcare?
War is largely mass murder with excuses - sometimes offense, sometimes defense; but we seem to be more tolerant of it than of individual murders.
The OP's question also raises the issue of the sociopath - who might really have this question, as not understanding what the big deal is about killing.
And for 'entertainment' look at how much people like killing in movies and tv, with whole genres around it, whole series about it - westerns and mod films, countless crime shows.
It's also very relevant in terms of global poverty - where inexepensive problems to fix kill many people, but the wealthy societies are reluctant to give to save lives.
It's an educational moral issue - whether to value human life universally or not.
And it's an issue as well about how people tend to give more weight to people closer to them - family, friends - then who are in their city and country - before foreign lands.
How many people are seduced to war by desire for 'feeling superior', for expressing hate or contempt for others, for the sheer excitement of 'winning a war'?
Less nowadays, when war has been almost indusrialized, so that we know abstractly there are wars in a place like Afghanistan, but people don't feel all that much 'exceitement' from it, it's not as if we're 'establishing our society is stronger', that's not in question, it's not as if there are constant victory rallies getting people excited, it's just war that serves some interests - oil industry, military industry, military bureacracy, and so on, and most people seem pretty bored and uninsterested.
And there's what to do about others' wars - say, the killing in Syria, support for the UN as a strong force to use aganst unjust war.
So the OP's question has a lot of related questions that don't seem as obvious for many people, who don't pay a lot of attention to a lot of issues related to killing.
A strong answer to the OP's question can underly a broader set of political values.
And it's one also to recognize in issues like gangs, noting, where is their moral education to appreciate why it's wrong to kill?
There was that recent news story about two 18 year olds who wanted a woman's cell phone and killed her when she didn't hand it over. Think they didn't have a good answer for the OP why killing is wrong? How do we get more people like that to have a better answer why killing is wrong so they don't do it?
How much do right-wing policies putting property rights before the needs of people contribute to a culture where killing is more accepted like the OP implies?
Finally, this issue comes up a lot in conflicts between 'the people' of a country and the powerful in the country (government and the wealthy) where there are 'revolutions'. Where do you draw the line between killing 'the people' when they revolt as 'criminals' or 'terrorists' and when are they justified to demand the fall of the government?
It's unthinkable in the US for there to be a violent revolution, given the sophistication of our military forces. More and more, the people globally face ever stronger forces.
This raises that issue, what is to be done in the face of tyrany, when tyranny is stronger and stronger?
It demands more and more protection for political protection for the democratic rights of people - yet we're doing the opposite, as money plays an ever larger role in politics.
People may think the OP's question is obvious, when it comes to why two people shouldn't kill each other - but is it as obvious to people on the other issues I mention?
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