I read the book in two different paces. The first part was so rich of philosophical material, so I read one chapter at a time and ruminating on it, and the second was a really captivating second hand embarssing narration that I couldn't put off and I read it as fast as I can with breaks between chapters.
In the first part, I felt like I really was the Underground man, I am that spiteful vicious creature who looks down upon others as inferior because they aren't as conscious of the world as I am. This was me for most of my life, I disowned my religion, my society and everything that ties me to a particular group because I knew I was better than that. Even though there is something true in it, deep inside I felt that maybe it was me who had it wrong, maybe it IS a disease.
All of the first part deeply resonated with me.
In the second part, narrator recalls his memories, his most deprecating memories, from 15 years ago. And it was really really difficult and relatable read I have ever read so far. It is 15 years later he is writing this. Readers can understand what he is doing wrong, HE knows what he is doing wrong ,by now, and is punishing himself for it. He is not some psychopathic person who is oblivious to his wrongdoings while calling out others' , he is just a hyperconscious man who knows about his wrongdoings and still failing to do anything about it.
We can say that he is a product of his society and it is interesting, but we should also understand that we are also the product of a similar society. The downfall of UG man is he is trying control the world around him by his will.
I think what Dostoevsky trying to say here is that one cannot change the world as one wants to, there is no equality in the world, there is no crystal palace in the world. He does not advertise us being the average thoughtless man, but he doesn't encourage being drawn into the void and question everything, essentially becoming nihilists. Rather he explains that emotions are real, it is out there in the real world too, rationality and emotions should go hand in hand and one should love oneself to love others. (This final paragraph may be a influence from his later works.)
I don't think I have conveyed what I wanted to say, but yeah, that is pretty much it.