My question is:
How can we prove that something is ethical, when ethical has a meaning to each human?
First, if you're looking for us to 'prove' things with absolute certainty (as in mathematics), it should be clear that that is not going to work in ethics. But note we can't get absolute certainty in science either: the core of scientific reasoning consists of generalizations and inferences to the best explanation, neither of which is deductive. So if you're looking for any kind of certainty, we can't even get that in science, let alone ethics.
This may seem like a small point, but I have seen far too many people hold a kind of false dilemma to the following effect:
"Either we can prove things with certainty, or else anyone's opinion is just as valid as anyone else's"
Well, I assume that you would consider science to be an area where it is not true that anyone's opinion is just as valid as anyone else's (or that any theory is just as plausible as any other), even though we also do not prove things with absolute certainty in science either. In other words, there is a wide area between absolute certainty and complete subjectivity; an area where we can certainly argue that certain ideas, viewpoints, or theories are better than others.
But yes, I sense that you don't recognize this wide area between absolute certainty and total subjectivity, and instead buy into the kind of false dilemma I expressed earlier. That is, you say things like:
"what I consider ethical, others don't"
"ethical has a meaning to each human"
which suggest that you see ethics as a matter of pure personal opinion; as if ethical questions are not any different from questions like: "which flavor ice cream do you prefer?"
I find especially telling your:
P.S Of course there are many things that are agreed upon by most (murder, cruelty, inhumanity etc), but I'm not discussing them, I'm discussing ethics in a social way.
Here, you seem to try and make a clear-cut distinction between two types of ethics: ethics where there is a lot of agreement (i.e. some 'objectivity') ... and ethics where it's all just a subjective, social, relativist crap-shoot
But, ask yourself this: on what ethical issue is there really no agreement at all? I mean, even with ice cream flavors we find agreement ... I doubt many people would find Frozen RoadKill to be the best flavor. And when it comes to any of the typical ethical debates (abortion, euthanasia, etc.) where it looks like there is a lot of agreement, please realize that there is actually a lot of agreement if you look a little closer. E.g. if we look at abortion from the standpoint of the mother who wants the abortion, we all understand that the abortion would be a good thing. But, from the standpoint of the fetus, not so. So, what we really disagree on, is how to resolve these conflicting values. But on the specific values themselves, we actually find a ton of agreement: we all value freedom, happiness, health, etc. etc.
So, when it comes to ethical issues, we can take those commonly shared values, and use those as a basis to argue for ('prove' in the non-mathematical sense) things.
Also, the fact that not everybody will agree to certain values (i.e. it is not 100% universal) should not stop us: the strength of the argument can be judged relative to the level of agreement to the premises. And again, the case here in ethics is not any different than in science: if scientists make an argument on the basis of the theories of evolution or climate change, well, even among scientists not everyone agrees to those theories. Hell, even some mathematicians question the use of the reductio ad absurdum.
In sum, I urge you to move away from the false dilemma, black-and-white worldview that you apparently share with (unfortunately!) many other people. We can make arguments in ethics, and argue that certain ethical positions are better than others, even if 'proof' (in the strong sense of the word) is unavailable.