Why is killing seen as bad...

Because killing is how social progress is achieved in most cases.

The neoliberal scum want us to be docile "pacifists" because they know that will keep us out of power.

Obvious OP is trolling. And Holla Forums defends genocide in nearly every thread, no matter the subject at hand.

Killing is "bad" (not in our self-interest) because it is the ultimate supression of the self and ego. Killing another person is not conductive to forming a union of egoists and thus outside our self-interest as socialists.

Why is OP wrong though?

Killing is only seen as bad when the media tells people that it's bad.

Killing ISIS? good.
Killing Iraqi civilians? They were probably secretly terrorists
Killing criminals? they deserved it
Killing peaceful protesters in Saudi Arabia? Self defense
Killing conscientious objectors in Japan? Necessary collateral damage
Killing kids? The enemy were obviously using them as human shields
Killing American soldiers? EVIL! OUR BOYS DID NOTHING WRONG!

Serious answer to b8:

Killing is seen as bad not because of some arbitrary belief in the inherent sanctity of life. Even those who believe that it is in their interest for certain people to be killed do not advocate for unpunished murder in the streets.

Nor is it because there is something inherently bad about death or being dead, if it's sudden and painless. The process of dying in its final stages is in fact an euphoric experience, and it is better to be dead than to be alive.

Killing is stigmatized because of two commonly held fears: the fear of death and the fear of suffering. Both are irrational fears that we experience only because our ancestors would have been unable to survive and sire our lines without them. The only world that exists is one in which we fear death because that is the only world in which we would be able to exist.

To put it simply, killing is forbidden because most people believe it in their personal interests for there to exist a system which has provisions in place to prevent their own deaths. Even if we accept that the fear of death is an irrational part of our natures that can be overcome and that death itself is not so bad, suffering remains a factor (unless you somehow reject suffering entirely, in which case you are already dead).

Now, if you do it properly, killing does not lead to suffering - given that the dead cannot suffer, it can more properly be said to end it. A botched killing can be very traumatic, naturally, and that's one more reason why we might look unkindly upon it, but even a killing that results in no suffering to the victim herself still causes harm outside of the victim.

To be specific, unless the victim had no contact whatsoever with other human beings and the killing was done in such a way that it would be impossible for it to ever be discovered, the killing causes a twofold suffering to the community of the victim - the pain of loss and bereavement, and the fear that they too could be killed. The victim was someone's friend, someone's child, someone's parent - being deprived of that individual's presence directly hurts the interests of other individuals within the greater community of which the victim was a part of. The stress of fear of death is invoked naturally as the individuals connected to the victim are sharply reminded that the same could happen to them.

Of course, this is why we introduce labels and qualifiers that distance the victim from others - he was a criminal, he was a kulak, he was a degenerate, he would have shot all of us first if we didn't do it, all so that we deaden our empathy and come to believe that the killing was actually justified. Which may well be the case - there's nothing wrong with killing someone if failing to do so would result in even greater suffering.

Thus do we prohibit killing. Of course, if you follow this line of reasoning to its natural conclusion, you realize that there's nothing under this scheme to stop you from suddenly and painlessly killing someone without friends and family so long as you make sure that the killing remains undiscovered and thus causes no suffering, unless you fear death yourself.

This is very much why it is in the individual interest to be connected to others and for others to be aware of what their interests are. When killing is done in defense of spooks, it's possible to be killed even when the killing is in absolutely no-one's interest. When you kill and exploit for your own personal interest and trample on those of others, you invite retribution and your own killing.

Ultimately, you could say that killing is bad because it's probably against your interests, most of the time.

read lord of the rings, gandalf explains it in one sentence, which i'm not gonna spoiler now.

also evolution makes animals not want to kill other animals unless they need to.

Love for the Proletariat
Hate for the Bourgeoisie

because people don't like to be killed, generally

best logical point
all those years growing up are wasted if you kill the elderly, and death squeals require energy (i.e. wasted carbohydrates which need to be recycled again)
this is why abortion is preferable, start em young

There

"haha im going to go out and kill fascists!"

"no, no, please have mercy i was only acting ironically no!!"

Why do you intend do kill? If it's mostly the proletariat, why should the proletariat support you?