The overall message that Solanas has is that men are the lesser sex, and that men have to keep women down because they want to be women and if women start living up to their potential they will realize men are useless and get rid of them. This is why men have created things like social expectations, economics, patriarchy, etc.
She also has some frankly bizarre views on the psychology of men, saying that men love death because they are already dead inside, and that all men want to be women, and the only men who SCUM should generally accept are the ones that embrace the fact that they are less then women and accept that they want to be women.
She essentially takes the sexism that women have faced for centuries and flips it, claiming that women are better and men are lesser and that the sexism we face is actually a construction to hide this fact.
Additionally, her writing is just not all that good, and she doesn’t really back up any of her arguments, just stating them as fact and ranting about it further. Her organization is also a little strange to me.