When a standing zombie is neutral they should roam around slowly hit their head gently on a wall, and groan a bit. When a zombie is agrodo they should look at you for .5 seconds, and then run at you with their head and body shacking like in 7 days to die so their head is harder to hit.
If the zombie is a crawler or runner thing they should roam around slowly and moan. When a runner is argroed they should act like the runs in 7 days to die, but be more clumsy.
Zombies should die to 1 shoot head shoot if nom armored, and 2 shoot head shoot in the head if armored. They should also take 3 shoot body shoot if nom armored, and 4 shoot for armored zombies. If shoot in the limbs they become like the many post people made.
I think to make zombies harder to hit, they should have variable speeds. This could be as simple as varying the speed of a single zombie as it walks or making it unstable, but you could also go the whole 9 yards with stumbling, limping, lunging, and brief dashing. This would make zombies feel more alive, and also make them harder and stronger in groups.