Why did Hashem offer the Torah to the non-Jews and why did they reject it?

There is a famous Gemara/Medrash that Hashem offered the Torah to the various nations of the world and they of course rejected it. The Medrash relates that the בני עשו rejected the Torah because it prohibits murder, the בני עמון ומואב rejected it because the Torah prohibits עריות.

This is very difficult for 2 reasons:

1. Murder and עריות are prohibited for all non-Jews even though they rejected the Torah because these are part of the 7 מצות בני נח. Additionally, why would Hashem specifically tell them these mitzvos which are not specific to the Torah but are part of the 7 מצות בני נח?
2. Murder is prohibited by basically all cultures including the בני עשו, so why would they reject the Torah because of it whne they actually agree with it?

It seems to me that the purpose of this Medrash is to bash the גויים and portray them as immoral people. This attitude has permeated orthodox judaism. R' Yechiel Yaakov Weinberg (the שרידי אש) wrote the following in a letter to Professor Samuel Atlas:

The entire world hates us. We assume that this hatred is due to the wickedness of the nations, and no one stops to think that perhaps we also bear some guilt. We regard all the nations as similar to an ass. It is forbidden to save a Gentile, it is forbidden to offer him free medical treatment, it is forbidden to violate the Sabbath to save his life
...
Can the nations resign themselves to such a deprivation of their rights? It is permitted to deceive a Gentile and cancel his debt as well as forbidden to reurn his lost object.  What can we do? Can we uproot our Torah teaching with apologetic formulae or clever deceptions?

It is very hard to argue with anything that R' Weinberg said.