If you think about it there are very few zombies in the whole history of zombie films who stand out as having unique personalities that take their character beyond the basic undead status. Whereas much of the horror involving werewolves, vampires and the like gets into the struggle between perceived sanity and perceived insanity. It's a battle of wants vs. needs, and the character's depth (or lack thereof) and personality will be reflected in that. I can only think of a couple examples of that in the zombie genre. The strong personalities come from those who are still living and still fighting.
I hadn't thought of it before, but this might be important, too. 'Transformative monster characters, for example, the ones who remain human but are in the process of becoming something else (human->vampire, human->werewolf, etc.) also have a theme of coming to terms with change. Adapting to a new way of being, and how change can either cause us to become nihiistic predatory monstrosities or something different, yet still human and good (as can be seen by 'heroic' vampires, werewolves, etc. in fiction). It shows that change need not necessarily be feared, but if you're strong enough, with the right will, personality and spirit, even a horrific transformation can't change who you are.
Zombies don't have that.
Think about it for a minute. The tranformation into a zombie is complete and total. It doesn't matter what you were in life, nothing survives. Your memories, your identity, any will or spirit that you had before--gone. Just a shambling lump of predatory intent with no redeeming value.
This makes it easy and morally unambiguous to kill them with impunity, but it's also informative of our attitude toward change and transformation, that it will inevitably destroy everything about what we are, and cause us to be debased creatures, sapped of our humanity... in the end, when you think about it, it's a very right-wing, conservative mythology, especially when you think about how this is probably the only genre where a gun-toting survivalist can actually be considered a 'hero'.
The only exception I can think of is Nick Frost at the end of Shaun of The Dead. He seemed to retain a good portion of his former self.
Heh.
More or less.