A hopelessly bad-tempered aspiring cop in a crime-filled city is bitten by a vampire, giving him the supernatural powers necessary to defeat the criminals overpowering the local police force.

The central idea of a cop or a virtuous personality becoming a vampire, who by definition must kill to survive, is an interesting one. If he's a rookie / aspiring cop, then 'hopelessly bad tempered' doesn't help. I also think the struggle within him, about wanting to be on the side of the law, whilst having a secret 'illness' is worth mentioning.

I like the idea of the cop getting a "super power" by becoming a Vampire. Your logline is succinct and clever, leaving me wanting to know more. Very good!

You could probably get away with removing "hopelessly bad-tempered," since it's not necessary to know for the basic plot. I like it otherwise and tells me exactly what it is. I think removing that one chunk will make it shorter and more concise.