Sorry but there is no 'objective' good. Good is defined by the society you live in. We believe slavery is an evil but many of our founding fathers did not. Were they evil or just used a different scale? If you were a cannibal you'd might think eating your enemy was a good thing.
OK, if there is no objective good, then you can't claim evil exists.
If there is no objective good, then what Jeffrey Dahmer did was not evil or wrong at all. In fact, without an objective measure, what Jeffrey Dahmer did is morally equivalent to an act of kindness, respect and compassion. One act cannot be better than the other, if there is no objective good.
Evil is relative to the society you live in. If you kept someone in slavery you'd be considered evil in our society, which is why it is against the law. 2,000 years ago keeping someone in slavery was admirable as it indicated your elevated social status.
No, that is not how truth and morality actually work. Just because a particular society or country believes something is true doesn't make it true.... in the same way that just because a person believes something to be true doesn't make it true.
Truth is not dependent on us at all. It just is what it is. I can believe 2 + 2 = 5, but that doesn't make it true simply because I believe it.
Let me ask you a question. If there was a society who believed that brutally raping children was morally OK, does that mean what they are doing is actually OK simply because they believe it?
According to your moral relativist position, what they are doing IS acceptable, simply because they believe it, in their society. And according to your moral relativist position, we have no right to tell them they are wrong when they brutally rape children, because their morality is no better or worse than ours, they are morally equivalent, if there is no objective measure.
It makes morality utterly meaningless, at the end of the day.