If there were no good in the world, why would we be having this discussion? Would it be only because the appearance of good is useful?
As a rhetorical question only, I suggest you look inside your own self. It is true we cannot fully know another person's motivations, but there is compelling and solid reasons to assume there are people with sincerely compassionate motivations. Looking inside yourself, can you find any inclinations towards compassion that are free from immediate benefits? Are you moved to lessen someone's suffering? Do you ever desire strongly to make another person have a good feeling? I've seen plenty in you to support the idea of goodness in people. I'll leave the personal note in the realm of the rhetorical now.
If one looks deeply enough inside self, it is possible to see a microcosm of humanity. There is some sense of both its cruelties and compassions. Our genetics and environment places outer boundaries on who we have the capacity to be, but we all have some elbow room to self-determine our nature. That is at least my belief based on what I know.