Q:

How to fix armature distorting my mesh?

I made a simple model with multiple meshes. One of them is the head The rest is unimportant for this question. The model has one armature which is the parent of the meshes. All the vertices in the head mesh are in one vertex group which has the same name as the head bone. The head mesh has no Mirror modifier. I set the pivot of the head mesh and all the other parts to the middle of the head mesh. I used armature from the add human (metarig) command. I applied all locations, scales and rotations. I have no idea what other detail I should supply.
Normally it looks like this:

However, when I try to rotate the head (not the neck, the head) it looks like this:

Can anyone help me on how to fix this, or should I actually rotate the neck?

A:

I think the distortion may appear, because the vertices of the mesh are improperly assigned to the bone. 

Select the object you want to be affected by the bone, enter Edit Mode and in Vertex Groups panel select neck group and click Remove.

Select the head group and click Assign.

The maximum weight of the mesh is assigned to the head bone, so now all the vertices will follow the bone completely (you can see the weights entering Weight Paint mode).

Now it should work fine.

