You could add a script to your zombies, e.g.:

public class Zombie : MonoBehaviour
{
    public int points = 1; // You can edit this in the inspector

    void OnDestroy()
    {
        PointCounter.points += points;
    }
}

The GUI script would then be:

public class PointCounter : MonoBehaviour
{
    public GUISkin guiSkin = null;
    public static int points = 0;

    void OnGUI()
    {
        GUI.skin = guiSkin;
        GUI.Label(new Rect(0.0f, 0.0f, 128.0f, 32.0f), points.ToString());
        GUI.skin = null;
    }
}

So, whenever a zombie is destroyed it will add the points its worth to the pointer counter.

GUISKIN: Read the answer below...

Just going to create a new answer for the comment:

... but how can I add a font or a texture to it? ...

You can use a GUISkin for that. You create the GUISkin in Unity in Project Hierarchy, then in the GUISkin you can choose label. In there you can set a font, size, and so on.

You use the GUISkin in the code by adding: GUI.skin = mySkin; in OnGUI() just before the label. You can then add the GUISkin by creating a public variable in the top of PointCounter with points. Then you can drag the GUISkin on to the GUISkin variable in the inspector.