Dane is a dominant omega, and he wants to hide it, since it can be dangerous for him if it is known. And Grayson already found out.
In addition Dane is a tsundere and he is "allergic" to love/relationships, because of a family related trauma.
I think reading kiss me liar and kiss me if you can will give you more of a perspective on why Grayson is so hated by all the good characters. He gave his (alpha) brother lifelong trauma by making him think that he spent his rut with a dog. And was always discriminatory against betas. Dane seems to have a strong sense of justice seeing how much he cares about protecting the weak and how even the thought of Grayson having been abused is enough to soften his heart.
I don't think I'm even feeling this story. Why does Dane even hate Grayson so much