Why did Rodger kill his roommates?

[SoCalNative]

5. It's in his written manifesto

Two new housemates moved into my apartment for the Autumn semester. They were two foreign
Asian students who attended UCSB. These were the biggest nerds I had ever seen, and they were both
very ugly with annoying voices. My last two housemates, Chris and Jon, were nerds as well, but at least
they were friendly and pleasant. These two new ones were utterly repulsive, and one of them had a very rebellious demeanor about him. He went out of his way to start arguments with me whenever I raised
the issue of the noise he made. Hell, even living with Spencer was more pleasant than these two idiots. I knew that when the Day of Retribution came, I would have to kill my housemates to get them out of the way. If they were pleasant to live with, I would regret having to kill them, but due to their behavior I now had no regrets about such a prospect. In fact, I’d even enjoy stabbing them both to death while
they slept.