Hard to kill

You feel bad about killing stuff.  Not talking people, yet, just things.

There was a dying rat in the neighbors yard a couple years back.  It's a RAT.  Yuck.  And it was dying.  Which is why it was in middle of the yard.  Prolly the endgame of the poison from the traps.  I dispatched it with a shovel, but I still felt a little pang for the thing.

A mad dog?  You shoot that.  But it is still a domestic dog and had the potential of being a loyal pet and companion before this.

Deer are cute and cuddly looking, but I have personally shot them.  Felt a tiny bit bad, but didn't hesitate much.  And it was delicious.

A tiger is big and majestic, and you hesitate to destroy that because it is so.  But you don't hesitate too much if it is coming for you and it is you or him.

There is a hesitation to shoot zombies, too.  Because they were people once.  Friends and family and neighbors.  Some mother's baby, once.  Fortunately zombies have few other redeeming factors apart from that.  They are deadly dangerous, they are revoltingly disgusting.  And that makes that hesitation, that pang of guilt, go down faster.