If they "organized" it, but didn't actually commit the murder, then any violent response is proportional only to an action (actual murder, not just offering money to someone else to commit murder) which the target of the response never took. Naturally, if you chose to "organize" a murder of your own in response it would be governed by the same rules, so you are free to respond in kind. However, anyone who chooses to accept your hit job would be acting just as aggressively as whoever chose to accept theirs; they could not claim self-defense, even as your agent, any more than the first party's hit man could shift the blame onto his employer.
What you are saying is that its wrong to forbid someone ordering a killing but perfectly ok to organise a killing yourself.
No, what I am saying is that ordering a killing does not justify actual killing in response. The topic of "forbidding" never came up.
So if the intended victim tries to retaliate, they are morally equal to the person who organised the killing.
That depends on the form of the retaliation. If the intended victim orders a reciprocal killing, this would be a proportional, defensive response. As such, they would be in a superior moral position, assuming one considers mere organization of a killing to be aggression in the first place. If they retaliate by personally killing whoever ordered their death, however, that would be a disproportionate response, and thus an inferior moral position.
Those who chose to accept either order would be morally equivalent; neither could claim to be acting defensively.
Even if you consider this "conspiracy" or "incitement" and/or a form of aggression, there is still the principle of proportional response to consider. How would one defend the claim that violence, of any sort, is a proportional response to a spoken statement, however threatening?
So even if someone walks up to you with a gun clearly in his pocket (assume that such gun carrying is normal and acceptable where you live) and says "give me $200 or I'll shoot you" it's not OK to attack him first? I'm all for free speech, but once you start taking things this far and outlawing preemptive strikes I really think we're entering into "evil will always defeat good because good is dumb" territory.
I'm not saying that you can't respond preemptively to an
intended action. In this situation the speech can be taken as a declaration of imminent intent to shoot you; however, you're responding to the shooting, not the speech. It would be a different matter if he said "give me $200 or I'll hire someone to shoot you".
Freedom of speech does not mean that you are free from the consequences of your speech.
On the contrary, that's pretty much exactly what it means: that speech, per se, has no legal consequences. (There may be other, extra-legal, non-violent consequences, but they are not relevant here.)
Intent can have legal consequences, and certain speech can be taken as evidence of intent, but that's not the same as being punished purely for the content of the speech.