The circularity happens when one starts looking for a definition of murder.
Say for instance, "Any purposeful killing of another human".. when it bumps into "I Killed him, because he was literally raping my sister in front of me, had my mother tied to an electric chair, and was actively electrocuting her while he climaxed, and my sister screamed in torment." The distinction between where one draws a line in the sand, and why, ultimately boils down to what is considered culturally acceptable (and MANY MANY things can be considered culturally exceptable. I do not want to take the thread down that rabbit hole. I am pointing out that the rabbit hole exists. This is not to be conflated, and I will not go down the rabbit hole to prove that it exists. I will leave that as an exercise for the reader. If you do not believe the rabbit hole exists, go investigate until you find it. It wont be that hard. The extreme example I just used is par the course for some of the things that can be considered culturally acceptable, and I DO have ready examples. Again, I do NOT want to go down the rabbit hole. I have simply looked down it, and seen that it exists.)
Since the distinctions all derive from the position one holds intrinsically as an embedded part of some culture, it is always evaluated from the relative position of that cultural fixture, and not taken in a vacuum. Taking that leap of cognition is not easy, and most people never try. However, once you realize that your opinions on the matter are just that, opinions, and have no bearing whatsoever on "validity" of a cultural view outside of your own, the circularity starts to become revealed.
See for instance, the western world's view on how the middle east treats women. For them, it is culturally accepted. For the west, it is abhorrent, and needs abolition. The position of the western world ultimately gets supported with notions of "It is better for the women, because reasons (that are tied to social dynamics, that are inherent to the position that I, the arguer am familiar with, and thus circular through the nature of a-priori implicit assumption that the familiar is better)-- to which the person from the middle east makes the same argument, and reaches the 180 degree polar opposite conclusion.
The rational observation is that this is occurring, and to seek to find some objective measure that exists outside of cultural morality for supporting or not supporting a particular platform. The problem, is again, that people do not normally take the necessary leap of cognition to escape from the implicit assumption of "my familiar cultural fixture is the superior one". (which is why nationalism, which enshrines that notion, is dangerous, and leads to exclusion by its very nature.)
One cannot have a rational discussion about murder without a uniform and rational definition of murder. (eg, is death from war murder? Kinda depends on who you ask, doesn't it. Isn't that the whole point of contention between neonazis and holocaust survivor families? One asserts that it was not murder, while the other does. The positions taken are from cultural positions of familiarity, and again, can reach 180 degree polar opposite conclusions using essentially the same formulae for the argument.) Since no such distinction exists, it is not possible to assert quantifiably if it is wrong or not. (see again, the extreme issue I presented with the really nasty guy who was raping somebody's sister and torturing their mother in front of them, and attempt to conceive of it in a circumstance outside of the cultural taboos you have been borne into. Some will assert that the death of that person is totally justified. Others will assert otherwise. Both assertions typically come from positions of cultural normalcy. This kind of thing is typically explored in science fiction, since within historic contexts, it gets tedious and heated very quickly-- again, as everyone stamps out their little fiefdoms of cultural familiarity, and batten down the hatches against being challenged against "Something so blatantly obvious.")
I personally have the following view (just to dispel attempts at strawmanning me into some kind of bizzaro bastard that needs to be destroyed, and thus proving my point about the above trend in argument tactic) on murder.
Murder (for me), is defined as the dis-equitable (in terms of total social benefit) taking of a human life.
Under this definition, the removal of the torture-rapist is seen as justifiable, because his continued activity would cause more net reduction in the total social benefit to the society than his removal; His removal contributes more to the local society than his continued existence.
This again, however, falls victim to the issue I pointed out-- It intrinsically is based on the position of a societal framework, and as already pointed out, all are fundamentally interchangeable. See for instance, how it falls victim to the specifically mentioned "Nazis vs Jews" angle. The very reason the Nazi's exterminated the Jews, was because they culturally agreed that the Jews were harmful to the Nazi society.
Any single position that you could take, results in this kind of collapse in validity. It is in this fashion, circular.