Mesh deforms when moved along x axis

Hi. I was following a tutorial about how to create a battle scene in blender. I created a mesh for fighter 1, applied the mirror modifier, duplicated the mesh to create fighter 2. And merged the two meshes into one. However when I move parts of fighter 2 around they change into weird shapes, but only when i move the objects along the x axis. I am doing this to create shape keys.

Any help would be much appreciated.