We are used to seeing cats as comically aggressive to mice in cartoons. Tom & Jerry just couldn't get enough of each other. I think they still kick each other around the screen on cable. An American Tail has cats as slightly more sinister, but ultimately you knew the mice were gonna be ok. Maus has this cast of characters, and is drawn as a cartoon, but has a completely different tone from either of these other examples.
The illustrations in Maus are so dark, and so serious. Bechdel was serious, but her book feels nothing like this. I think Spiegleman's lines look like he was pressing down very hard on his pen. It was not as planned as Bechdel - his illustrations looked almost rushed. I think this lends such a feeling of urgency to the story. Adding to this are his manic, addictive personality and his father's ill health.
Why do you think he wrote this book? What does his illustration tell you about his point of view? Do you think there is any truth to the McCloud idea that if you can make a character less specific they are easier to relate to? These are mice and cats and pigs - yet humans. This is not an accident. Why use them?