The title is based on the premise that there is such a thing as ethics: Right or wrong, good or bad, ethical or unethical, moral or immoral, and so on. The entire discussion is encompassed in a body of philosophy called moral relativism.
It's just like it sounds. Morals are based on how relatively malevolent or benevolent an action is, either to an individual, group or society as a whole.
Without using God and His commandment as a baseline, it comes down to the relative injury being caused, to the individual, the individual's family, and the community.
In Texas, they used to have a law that said a killing is justified if the accused can find 3 upstanding citizens to testify that the deceased "needed killing."
Taking a life is the most injury one can inflict. Therefore, on a scale of low-to-high relativity, it's at the top. That's from a pure physical perspective. In some cultures, to dishonor the family is relatively worse than death. Therefore, some cultures placed murder of one who dishonored you below other injuries. Suicide was used as a form of "apology" for failure to maintain one's honor.
So, on its face, the question of any act being "wrong" needs to be defined. "Sin" implies a religious context. Illegal implies a set of socially agreed upon rules. "Bad" implies a negative impact other than to the victim.
Plenty of discussions all over the web on moral relativity and "is murder wrong." Not sure regurgitating it here is productive use of everyone's time considering the volume of information and ideas involved.