If you are referring to "Is MURDER immoral?" then many people would be capable of arguing with a resounding YES. You used the phrase ". . . wrong across the board . . . " which suggests that you have some idea or notion what morality is to ask the question. We must be clear what morality is.
I submit to you that the proper term for some act deemed right or wrong across the board is MORAL. When rational people use the term or moral we are not referring to or expressing something subjective (i.e., a personal opinion, personal belief, personal thoughts, cultural beliefs, cultural up bringing and so on). I also want to further indicate what else the term or concept MORAL (or the topic of morality) is typically mistaken with; namely many people think that morality is about authority (i.e., a military General Officer outranks a Sergeant so what ever the higher rank says goes). Rank or authority is not what makes an act or behavior moral or immoral. So there should be absolutely no comparisons to the LAW. Because some act is legal or illegal does not necessarily mean the act is moral or immoral. Your supervisors at your work place does not make moral rules. He makes ETHICAL decisions or rules. Notice there is a difference there. Because a workplace has ethics or because a field such as medicine has ethical policies does not make them MORAL. Ethics committee members are authoritarian. They outrank you and me and others not making decisions. That has nothing to necessarily do with morality. Morality, as a field is supposed to be OBJECTIVE--not subjective and definitely not might makes right (authoritative). Morality is supposed to be objective. You don't have to have royal blood or a certain amount of wealth or a certain rank to make moral claims. Well what do you mean by OBJECTIVE?
OBJECTIVE Truth, for instance, is a term that expresses that a meaningful proposition is always TRUE without exception. OBJECTIVE here means the answer or solution is NOT based on human awareness or human knowledge. It is independently derived. Well what does that mean? It expresses that a proposition is or can be discovered INDEPENDENTLY of the human existence or human experience. For example, gravity can be discovered independently by different scientists. No one scientist has the upper hand at proving there is gravity. No one person has the copyright to gravity. No one person has all of the knowledge and is hoarding it from others. Also note gravity likely pre-dates human existence. So gravity existed before humans were aware of it. Objective is used by scientists a bit differently than I am here. For scientists their version of objective includes EVIDENCE which is why it fails. If physical evidence is required to prove something then obviously there can be truths we are aware of but have no proofs. So scientific knowledge is limited. Secondly, the term OBJECTIVE requires a truth value that NEVER ALTERS. Science fails here again. Evolution for instance has been proven objectively wrong several times. How do we know this? Because the original theory had to be modified several times which indicates some propositions were indeed falsified. So scientists then updated the original theory to reflect the change in truth value of some of the propositions expressed. Objective in science means something different than in Philosophy. Objective means or expresses a CONSTANT as opposed to a contingent truth. A contingent truth is a proposition that can be true today and that same proposition could be false at a different time frame. The weather is contingent: it is not ALWAYS RAINING or SNOWING in all places. So do not confuse CONSTANT TRUTHS with contingent truths. Morality is objective and deals mainly with objective truths. Again a moral claim is a claim that cannot later be proven wrong. The speaker was JUST WRONG about his claim. We don't change the value of the claim itself. You can't make moral turn into immoral or vice versa.
Morality by context usually implies the rule is applied universally (in all places) otherwise the term would have little relevance. If I were speaking of an act only locally I would not use the term MORAL. That would be a weird choice of using language. So if I say "Murder is immoral" in am not speaking of just where I am located it is immoral. I mean everywhere on this planet murder is immoral and if my claim is objectively true it is true always forever. If I change the topic the form of the proposition does not change: "Abortion is immoral" has the same form and treated the same way. "Act x is moral" or "act x is immoral" have the same form as well. Act x is moral implies always moral everywhere. Act x is immoral means act x is always immoral everywhere. Notice the basic form excludes emotive language and personal preference. There is no " I think act x is immoral", nor is there an "I believe act x is immoral", nor "I feel act x is . . . "
Now I have hopefully convinced the reader what morality is NOT, I can move forward. The idea of expressing a moral concern is there ought to be some objective truths relevant to the topic. So before one even utters some act is moral or immoral there should be some objective truths on their side. Moral claims are not to include subjective elements or emotive elements, remember? We see this violation in reality with the abortion arguments. What we need to resolve the abortion is immoral argument for instance is a clear definition of what a "human being is". Because there is no one universal definition that doesn't exclude members of the status human being the issue is still raging. Unfortunately the anti abortionists use emotional pictures in demonstrations and emotive language on the regular. They don't seem to have objective truths on their side. If they did perhaps they don't know how to present them which makes that side of the argument look worse. They are making a moral claim without objective truths. You can't make a moral argument without objective truths.
Murder is a term that typically has universal elements in the definition. The main essential elements are a human victim. A human victim is an absolute necessity. No human victim then there is no murder , period. There is also the concept of the victim was NOT accidentally killed. I don't care how the laws are worded specifically the concept is more important. A human died and this was not an accident. We are not talking about accidental death. There is also a human being that causes the other human being to die. We are not talking a train killed the victim or natural death of the victim. For example, Cain caused the death of Able and that death was not accidental. Therefore we say Cain murdered Able. I am NOT saying that is all the components of what MURDER IS. there may be more to murder but what you can't deny is these elements I mentioned are also present with others elements. So we can sum up the objective truths here about the concept of murder: a human victim must occur, another human being must cause the unnatural death of the human victim and we know the victim was not killed by accident. We could add more. Of course other human beings may word these truth differently but they cannot escape the meaning of the propositions. Different wording does not change what a proposition is. The essential meaning is what is taken from propositions not the literal sentences used in reality.
From here we can now offer reasons why murder should not be allowed universally. By universally I absolutely do not mean all people have to AGREE to the terms. Objective truth do not require human agreement as they are independently derived and morality is not always utilitarian (the greatest good for the greatest number). For instance, the victim likely did not choose to be killed at that moment in time. Now for those who say what if he did choose to die that would NOT be murder. The term euthanasia would be used not murder. Another reason could be that most murders are painful and the most people prefer to avoid unnecessary pain. The victim could have had future plans which would indicate he decided to live longer and not be killed unexpectedly. I could go on and on but these are a few to justify why murder should be seen universally a wrong act to commit. Then I would bring up this behavior left unchecked leaves all human beings at risk of being the next victim. That means the rich, wealthy, poor all alike can be the next victim. No one of any stature is safe. No authorities are safe --not even the President of any countryKing, or Dictator. We would have no need for security for authorities like the President, senators, mayors, etc. All of these reasons are unlikely to be refuted.
Again the concern of MORALITY is not that all humans agree or the majority of humans agree with what is stated. What is objectively derived can be sought by anyone with reason. There are no instances of favoritism, nepotism, discrimination or biases in morality. People may and typically do use the term MORAL to mean opinion or subjective in reality. This is because they don't understand concepts in general or choose to ignore the power of reason. These people prefer an authority in their lives to tell them what is leagal, ethical and then confuse morality with that constant authoritarian exposure. Some one has to be in charge! So to minds like this almost everything is subjective.