Okay, this manga is a masterpiece! The previews chapter made me think he was actually the one that tried to killed his cousin but now we can clearly see that he twisted the facts in his mind and that he is just blaming himself for everything that happened.
The typical " well, I went out with my friends when I know he doesn't like it when I do so ofc he got angry... is MY FAULT"
The whole "if I had died... " is heartbreaking.