A zombie is a zombie is a zombie, right? Sure, some of them are crustier than others. Some don't have legs, some are missing half their face, but they're all basically the same. They want your brain, you want to blow their heads off. Easy.
Ah, but Warm Bodies complicates things a bit. These zombies not only have their own personalities, they have names. Well, they have the first letters of names and they're desperately trying to remember the rest. There's something about a name that makes them seem more like people, and that just makes it harder to want to kill them.