Diary of a Seducer was an essay by Kierkegaard used to explain the aesthetic life. He wrote it shortly after breaking off his engagement with Regine Olson, which he said was because he could not commit to being a husband, and like the comic says, due to his profound depression. Rumors circulated that it was based on his own broken engagement, but that was unfounded. It's possible that Kierkegaard published it to try to make himself look a bit cooler though.
Not totally related, but Kierkegaard's journal of how he originally "seduced" her is pretty funny too (she was already engaged to Johan Frederik Schlegel at this point, who she eventually married):

On 8 September I left home with the firm intention of settling the whole thing. We met on the street just outside their house. She said there was no one at home. I was rash enough to take this as the invitation I needed. I went in with her. There we stood, the two of us alone in the living room. She was a little flustered. I asked her to play something for me as she usually did. She does so but I don’t manage to say anything. Then I suddenly grab the score, close it not without a certain vehemence, throw it onto the piano and say: Oh! What do I care for music, it’s you I want, I have wanted you for two years. She kept silent. As it happens, I had taken no steps to captivate her, I had even warned her against me, against my melancholy. And when she mentioned a relationship with Schlegel, I said: let that relationship be a parenthesis for I have first priority...she mostly kept silent.

"Yes, but what if the philosophy zombie can't feel any pain, and is only pretending to be a wimpy nerd?"

A philosophical zombie is a thought experiment where you imagine a human being who is identical in every way, including in behavior, except they have no internal experience of consciousness. It's highly controversial whether such a thing is even conceivable, and has been used by many philosophers, including David Chalmers, to argue against concepts like physicalism (i.e. that physical matter is all that exists and consciousness is merely a part of the physical matter of the brain).

"Does Capitalism encourage the worst men to have power? No, Americans are just bad."

Herbert Spencer was a libertarian capitalist philosopher, who thought the government should interfere as little as possible with the free market. Like most libertarian philosophers, he thought government intervention interfered with human freedom and our ability to make contracts with each other freely. However, he also believed that government intervention, such as with poor-laws (i.e. welfare) interfered with the natural process of evolution. He believe the brutal competition of capitalism was good, because weak firms would die out, which would lead to an improved society. Government should not assist firms that are financially struggling, since it only harms society in the long run by allowed inferior firms to survive. He also applied this logic to individuals, however, believing the humanity must undergo a "purifying process" to create better genetics that would be more suited to deal with the new industrial form of society. Eugenics was not an uncommon idea at the time, but Spencer was very enthusiastic about it, and is the philosopher most associate with Social Darwinism and the phrase "survival of the fittest", which he very much applied to humans. He more or less thought it would be good for the genepool if the poor died out, because they are obviously unsuited to flourish in the next stage of human history.
Andrew Carnegie was very pleased with Spencer's philosophy, because it showed that it was right and natural for men like him to be on top, and he wasn't evil or oppressive at all, but in fact driving society forward towards the better. He invited Spencer to Pittsburgh to see the advanced capitalist production that his ideas inspired. Spencer was extremely disappointed with Pittsburgh, finding the city to be dirty, polluted, the factories were oppressive, and the capitalists overly obsessed with competition, saying about Pittsburgh "six months residence here would justify suicide."
Carnegie, like the comic shows, gained a competitive advantage in part through brutally oppressing his workers, making them work 12 hours a day, 7 days a week (they only got the 4th of July off, to celebration the greatness of America), and forgoing safety measures to increase productivity. At one point they tried to unionize, and despite claiming to have never hired scabs or opposed unions, he left the country to avoid responsibility and make it clear to his underling to do whatever it took to stop it. Workers ended up getting killed, and the attempt to unionize failed.
You can read more about the real story here. https://www.pbs.org/wgbh/americanexperience/features/carnegie-herbert-spencer/
You can read more about Carnegie breaking unions here.
https://www.history.com/news/andrew-carnegie-unions-homestead-strike

Why do I despair? I do not despair as much as I should.

As an aside, of all the philosophy quotes I've heard, I always thought Simone de Beauvoir's "all men must die: but for every man his death is an accident and, even if he knows it and consents to it, an unjustifiable violation" would be the most badass one to say just before you shoot someone.

This is based on Simone de Beauvoir's novel All Men Are Mortal, about an Italian prince who takes an immortality elixir in the 13th century, and uses his immortality to try to create a completely rational, utopian world for several centuries before more or less giving up. He tells his life story to a French Actress that he falls in love with in the present day, in an attempt to once again inject some kind of meaning into his life. It's a bit of an under represented novel of the ones written by Sartre, de Beauvoir, and Camus, and really should be the one teenagers read instead of The Stranger, in my opinion. It's a lot more straightforward in what it means, and it has cooler stuff like conquering Florence and burning monks at the stake, which I certainly would have enjoyed reading more as a teenager than a bunch of talking about how hot a courtroom is.