It's like when a kitten tries to bite something to death.
Greg is comparing one of the rich girls in his school who freak out at the sight of urine on stairs even though it's a daily sight. Greg is surprised by the intensity of her reaction, and the fact that her reaction is hardly noticed by anyone enough to care. He is comparing her to a cute kitten who tires to attack someone. Even though the kitten may have the deadly instinct of a predator, she is hardly thought of as one and is adored by people.
So at the top of the Benson social hierarchy, there is a vacuum. The result: chaos
Greg compares the Benson High School with the school he has witnessed on TV and believes that like schools on television, his school must have a hierarchy of cliques which are further ruled by one supreme clique. He compares this lack of a leading clique to a vacuum. And, calls the result of this vacuum, a chaos, which is the normal high school life.
"Mitzvah" is Hebrew for "colossal pain in the ass."
Greg compares the task of befriending Rachel to a colossal pain in the ass, as this is something he is forced to do. He is a person with low self-esteem and has worked very hard to avoid being friends with anyone and still not being a social pariah. The act of befriending Rachel, especially because she is going to die, doesn't agrees with him as he is sure that she will through his ruse, and worse he will be started to be labeled as her friend at school.
Basically, it’s like a low-security state prison.
Greg compares the Benson cafeteria during lunch hours as a low-security state prison, as it is full of people who are trying to hit other people with food and brawls are started when they hit the wrong person. Plus, there is always same food, a lot of which is often strewn at the floor, making it sticky and disgusting. He believes the palace to be as undesirable as a prison.
I am the Thomas Edison of conversational stupidity.
Greg is an awkward teenager, with low self-esteem. He avoids getting into meaningful conversations with anyone. So, when Madison Hartner comes to visit him the hospital after he breaks his arm, she tries to console him by saying that he is doing a good thing by being with Rachel at this point. However, he believes that he is not doing anything substantial. He regrets it later as he could have used this conversation to make Madison like him.
