There simply is no definition of right and wrong, other than 'What one would believe to be a good thing to do, and what one would believe to be a bad thing to do'.
Like most people have stated here, it's down to the individual. There is no way we can say something is definitely right. It's not written as a law of the universe, it's not physics, it's not a definite thing. It's something that we make up as a society, and as individuals.
I may well say the right thing to do is, say, nuke France. If I did believe that to be right, then under the laws of language and ideas within society, it would indeed be the right thing to do.