in "Q Who?" the Borg do actually cut through the Enterprise and kill a chunk of crewmen.
No, they cut into the Enterprise
and take a core sample of the ship. The fact that there are people in that core sample is of no concern to them. They're doing it to study the technology.
(At least, that's the evident intent in "Q Who." In more recent novels from Pocket Books, taking the later retcons about the Borg into account, it was established that the 18 Enterprise
crew members in that section of the ship were assimilated. One of them is a character in my TNG novel Greater Than the Sum
The Borg also are stated to have more or less genocided Guinan's people in the same episode.
No, because genocide implies the conscious intent to destroy a race. The Borg assimilated the technology of Guinan's world; the people just happened to be in the way. That's the "Q Who" version of the Borg -- they pose a threat to living beings only through disinterest in their existence. They leave you alone as long as you don't get in their way, and if you are in their way, they'll steamroll right over you without even noticing.
A Borg civilization which desires your technology and will take it and also wipe our your race in the process is something you can tell more than one story about.
Sure, and you can tell more than one story about getting hit by a hurricane or a tsunami, but it would get repetitive after a while. All you can really do there is tell stories about the victims and survivors, because those are the characters. But the writers of TNG and VGR decided that wasn't enough. They decided they wanted to give the Borg a face and a voice so they could write about them on more of a character level, rather than just writing about people affected by the Borg. Did they absolutely have
to do that? No. Writers of fiction rarely "have" to do anything, except keep writing. There's always more than one possible choice to make. But the choice they did make was one that had understandable dramatic reasons behind it. Turning a faceless foe into a more personified foe broadens the story possibilities.
And it's not an unprecedented decision. Terry Nation did the same thing with the Daleks of Doctor Who
when he introduced Davros, their creator. He felt that Dalek stories had grown stale, so he added a villain with more personality to bring something fresh to the concept. And Stargate SG-1
did it too, when they introduced the humanoid Replicators after several seasons' worth of "metal bug" Replicators. It's a natural impulse for storytellers to want to tell stories about people rather than things.
And in the sense of being a faceless menace that cannot be negotiated with and will kill you, the Borg resemble zombies enough for the purpose of the analogy.
Not the analogy I was making.