Nobody likes Casimir. Did you catch that? We'll say it again: Nobody likes Casimir. According to Nina, this is because dude is majorly creepy and inherently unlikeable:
He would go on and on about 'the good old days' when you could buy your very own slaves and kill them with impunity. He would do the most awful things with his tongue, which was long and blue, like one of those poisonous jellyfish. He had eyes like oysters, and teeth like tombstones. (1.89)
Um, yeah—he definitely doesn't seem like a nice guy. When we find out that he's responsible for infecting all the vamps in the book, we like him even less. Shipped from Europe to Australia in 1907 disguised as a mummy, Casimir promptly bit Horace upon his arrival, who in turn, infected Sanford. On a later spree, Casimir infected Nina and Dave. Dude just doesn't know when to quit.
Also, his apartment is totally creepy and he sleeps in a coffin—or, slept, that is, since Nefley killed him by staking him. Nobody was particularly sad to see him go, but his death serves a purpose since it inspires Nina to get off her butt and do something so that she doesn't end up truly dead like him. We never get to know this terrible dude, but his death launches the entire story. So, um, thanks, Casimir, we guess.